If your Christmas presents have been rather slow to arrive, this enormous pile of parcels may explain the delay.

Thousands of presents remain buried in mountains of post bags at sorting offices, it has been revealed.

One image posted on Twitter displayed the huge pile of Royal Mail sacks in Bristol.

A second showed that sorting offices are so inundated that staff are having to stack items outside, putting parcels at risk of being damaged by rain or even stolen.

Union bosses are reporting similar scenes across the country and have described the Christmas delivery chaos as a 'nightmare' after businesses complained parcels were taking up to a month to arrive.

A massive switch to online shopping means there are an estimated 200million more parcels in the postal and courier system this year. Online orders are expected to be up by more than 50 per cent as internet festive shopping overtakes the high street for the first time.

The Royal Mail has hired 33,000 temporary staff to help deal with demand.

It comes as a scarcity of online supermarket delivery slots risks sending scores of Christmas dinner plans up in smoke.

Shoppers are desperately scrolling through slot times but finding they are fully booked or will not arrive until after December 25.

Asda, Sainsbury's and Morrisons all confirmed to MailOnline that they were grappling with a high demand for deliveries amid a festive rush.

Meanwhile delays with Christmas presents are threatening to ruin Christmas for families and businesses.

Many customers have complained about items arriving late, while John Lewis, Boots and HMV have blamed Royal Mail for delivery delays. Terry Pullinger, of the Communication Workers Union, said: 'We could not possibly have anticipated this level of packets and parcels, it seems to be intensifying every day.

'That coupled with the arrangements that are going to be in place to keep key workers safe because of Covid, and the rising spread of Covid, is complicating what is already a strategic nightmare.'

Ian Trehearne, branch secretary for Gloucestershire, said sorting offices have been 'flooded' and had been 'under the cosh' without let up since April.

Text accompanying the picture of the mountain of sacks, posted by a CWU official, read: 'Our members in Royal Mail are facing unprecedented workloads. They are doing absolutely everything they can to keep the country connected.' It added: 'Pandemic + Christmas + record traffic.'

Businesses urged Royal Mail to be more transparent around its delivery times so they can give more realistic expectations to customers, with one saying it had received 'threatening' messages from people angry their order had not arrived, reports The Telegraph.

A Government source told the publication that ministers were 'concerned' about the backlog but could not intervene because of Royal Mail's status as a private company.

A spokesman for Ofcom, the postal regulator, said customers should take up complaints with retailers.

Meanwhile the last-minute dash for supermarket produce also appears to have resulted in supply issues - which left one Tesco customer furious when her delivery arrived with no turkey.

Experts said that coronavirus had triggered a 'seismic shift' in the retail industry which is increasingly migrating online.

But as Christmas approaches they warned supermarkets capacity is stretched and stores are buckling under the demand for deliveries.

Kath Hedley, 76, from Sudbury, Suffolk, who had been using online deliveries during lockdown, said for the past three weeks she has tried to book a Christmas week slot.

'They always show as unavailable,' she said. 'So we will have to start shopping again.'

Her frustration will resonate with people self-isolating, now wondering how they are going to source food, and others choosing to avoid public places ahead of meeting relatives in the Christmas relaxation.

Posting a photo of her order, Tesco customer Jennifer fumed on Twitter: 'Just received my online delivery (last slot available before Christmas) and one product was not available with 'no appropriate substitution available'.

'Only the turkey!!! Really?! On 17 December no alternative turkey you could send? Beyond disappointed.'

Others complained at not even being able to book a time to have their order delivered.

The Communication Workers Union asked for sympathy for postal workers, saying serving the public was central to what they do. However, a request to 'show them some love' by writing positive comments did not bring the comfort and joy they were hoping for.

One person wrote: 'Not the best photo. I wouldn't be happy if my parcel was among that. Might show how busy it is, but what idiots thought stacking bags like that is good idea?' A second said simply: 'Get on with it, you slackers.'

However, one grateful Twitter user wrote: 'I have two different posties, both unfailingly courteous and cheerful.

'They are the backbone of a community at any time of year, but especially now at Christmas and in a pandemic.'

A postman's wife added: 'My husband has been a postie for 28 years. He's been starting at 2am instead of 6am every day for the past month, to get the backlog of parcels/ letters sorted and delivered.'

Royal Mail has taken on 33,000 seasonal workers but industry experts say this is still not enough to meet demand, and many retailers have responded by bringing forward their last order dates.

In a statement, Royal Mail said: 'Despite our best efforts, exhaustive planning and significant investment in extra resource, some customers may experience slightly longer delivery timescales than our usual service standards.'

Several small retailers said their parcels have been delayed by as much as four weeks.

Tory Fox-Hill, founder of Fox and Moon, a Leeds-based stationery company, said hundreds of parcels sent out on the weekend of Black Friday still have not been delivered.

'Royal Mail are saying there's an unprecedented amount of post and there are slight delays. But obviously there aren't slight delays, it's three to four weeks,' she told The Telegraph.

Martyn James, of consumer complaints service Resolver, said Saturday was realistically the last day for shoppers to order online and hope to get their presents in time.

In an letter posted on Instagram, The Slabb, a chocolate shop in Leamington Spa, said it 'cannot understand' why Royal Mail would tell the public to expect only 'slight delays' when the reality in its experience is different.

The letter read: 'We, the small business owners, are being sent let's say 'colourful' messages, being sworn at and threatened.'

Richard Lim, chief executive of Retail Economics, said that the pandemic had caused a sea change in how the nation shops.

He told MailOnline: 'The impact of Covid-19 has seen a seismic shift, with customers shopping online.

'What's interesting with food is that it seems to be the part of the (retail) sector where there's been a permanent shift.'

He explained that, unlike other retail outlets which regained physical footfall when their stores opened after lockdown, the change in food shopping behaviour has been 'sticky' and the use of online deliveries has not regressed since restrictions eased.

Yet he warned that supermarkets 'are limited by capacity, and it takes years to alter delivery services' such as shoring up warehouse and online infrastructure.

Boris Johnson has relaxed coronavirus restrictions so that three households in England can mix between December 23-37.

The temporary easing of curbs has put family gettogethers back on the menu - along with traditional Christmas dinners.

As a result, online delivery slots have been snapped up and left many unable to book an order that will arrive in time.

Ryan Peach tweeted Tesco, saying: 'There are no delivery slots available and I'm having to isolate are there any options available?'

Karen from Tesco responded: 'Hi Ryan. I'm sorry that you are struggling to get a slot. I'm afraid that as these are fully booked, I'm unable to assign you a slot or make a slot available for you.'

Mr Peach was left deeply unimpressed: 'That's a shame. You would think when you can't leave the house that Tesco would be able to organise a slot!'

Another person wrote: 'I have to go to Asda and gosh I'm worries it's going to be busy and no social distancing. But I need to get food and there's no delivery slots online.'

Another said sarcastically: 'Well done Sainsbury's - not a single slot available until December 28. You are bloody useless!'

Tom Ironside, director of business and regulation at the British Retail Consortium, said: 'Retailers are working as hard as they can to fulfil the huge increase in demand of online delivery requests this Christmas.

'The ongoing pandemic is putting enormous pressure on delivery across the country, but as always firms will to what they can to address and rectify any issues as soon as they are made aware of them.'

A Sainsbury's spokesperson said: 'We've seen unprecedented demand for our home delivery service in the run-up to Christmas.

'This year, more than ever, we strongly encourage communities to support one another during the festive period by sharing deliveries and shopping for others where they can.'

An Asda spokesperson confirmed to MailOnline that delivery slots were 'few and far between' and that, because their lorries are not dispatched on Christmas Eve and Christmas Day, Wednesday would be the cut-off to get a booking.

A Morrisons spokesperson said: 'At the moment there's limited slots available online at Morrisons.com.

'There's more ways to shop at Morrisons this Christmas including Click & Collect, Amazon or Deliveroo as well as Food Boxes delivered direct to your door or Doorstep Deliveries where an order is placed over the phone, and delivered via your local store the next day.'

A Tesco spokesman said: 'We work hard to provide great availability for all our Festive Food to Order range items including turkeys.

'This range is available for delivery from 20th December as it is fresh produce. We were sorry to hear that Jennifer did not receive the frozen turkey that she ordered and we are working to resolve this for her.'