Border Force workers, ambulance crews and DWP staff among those joining Christmas strikes this week
Thousands of flights could be disrupted during one of the busiest times of the year as Border Force employees walk off the job.
Strikes will continue across a number of industries this week in an escalating campaign of industrial action, with Border Force workers among those expected to walk out.
The strike by more than 1,000 Border Force employees will affect passport control desks at Heathrow, Gatwick, Birmingham, Cardiff and Glasgow airports from Friday until Boxing Day, and then from 28 December until New Year's Eve.
More than 10,000 flights are scheduled to land at those airports during those times and it is feared that the strikes could see people waiting more than two hours in passport queues during the peak Christmas holiday period.
According to a report in The Times, contingency plans are being drawn up that would see passengers held on arriving flights to prevent overcrowding in arrival halls.
The newspaper quoted a source involved in discussions as saying: "Border Force are talking about moving staff around the country but it is a very busy period.
"Delays of two hours at the border are being routinely discussed in meetings.
"If everything backs up, or anything fails (such as e-gates), then airports will have to instruct that passengers are held on planes to prevent overcrowding."
Some airlines have already taken action to limit the fall-out.
On Friday, it was revealed that British Airways and Virgin Atlantic are limiting ticket sales for flights to Heathrow during the Border Force strike days.
Both airlines are letting customers move their travel to a non-strike day, and they are also keeping schedules and ticket restrictions under review.
Ambulance workers will also begin their industrial action this week, with 10,000 staff in England and Wales expected to walk off the job on Wednesday.
All three main ambulance unions - Unison, GMB, and Unite - will be involved, while GMB members will strike again on 28 December.
The government has announced plans to deploy 1,200 soldiers to staff ambulances and passport control, but union bosses have said the military are not "sufficiently trained" for these roles.
And head of the armed forces, Admiral Sir Tony Radakin, told The Sunday Telegraph that the armed forces are busy and need to "focus on our primary role".
Meanwhile, Royal College of Nursing members will strike for a second time on Tuesday in parts of England, Wales, and Northern Ireland, with the union saying the government has 48 hours from the end of the strike to agree to pay discussions - or face more extensive industrial action next year.
Health Secretary Steve Barclay has continued to resist calls to negotiate on pay, instead suggesting discussion about other issues that "matter to staff".
He has said the nurses' pay demands are "not affordable" and that the government is accepting recommendations from an independent pay review body "in full".
Other strikes this week include rail workers, driving examiners, highways workers, postal workers, and Department for Work and Pensions (DWP) staff.
Members of the Public and Commercial Services union (PCS) in Liverpool and Doncaster who are employed by the DWP will take action from today until Christmas Eve and again from 29-31 December.
PCS general secretary Mark Serwotka said: "The first week of our strikes has already caused disruption to farmers awaiting payments, learner drivers waiting to pass their tests and those using our roads - and it's only going to get worse unless the government puts some money on the table.
"Our members carry our important jobs, keeping the country running, and deserve much more than the 2% pay rise they've been offered."
A DWP spokesperson said: "We greatly value the work of our staff but the PCS union's demands would cost the country an unaffordable £2.4bn when the focus must be on bringing down inflation to ease the burden on households, protect the vulnerable and rebuild our economy.
"Benefits, the state pension and child maintenance payments are paid automatically and people who rely on that support will continue to receive it."
The Cabinet Office is expected to publish a new "resilience framework" today, bringing together all levels of government and including the private sector, charities and the public to "bolster" the UK's preparedness for industrial action.