The row over Tory sleaze reached new heights on Saturday night as MPs demanded details of any lobbying by Owen Paterson of government ministers on behalf of a company that won almost £500m of Covid-19 related contracts last year.

The crisis facing Boris Johnson also worsened after the former Tory prime minister, Sir John Major, described his successor’s attempts to block Paterson’s suspension from parliament last week for breaching paid advocacy rules as “shameful”.

A new Opinium poll for the Observer shows ratings for Johnson and his party have slumped dramatically since last weekend, with the prime minister’s personal approval figures hitting their lowest ever level.

With Tory MPs already fearing their party is regaining its reputation for financial impropriety after last week’s chaotic events involving Paterson, all the main opposition parties turned up the pressure.

Labour, the SNP, the Liberal Democrats and the Green party all switched their focus to the award of pandemic contracts, demanding investigations by the cabinet secretary, Simon Case, or the parliamentary commissioner for standards, Kathryn Stone, into Paterson’s contacts with ministers during the pandemic.

Paterson, who announced his resignation as an MP last week, was paid more than £8,000 a month for 16 hours’ consultancy work by Randox Laboratories, until he resigned from the role on Friday. Randox was awarded two Covid testing contracts last year worth nearly £480m without the normal competition. Government officials cited the urgency of the pandemic as grounds for not advertising the contracts.

The company insists Paterson “played no role in securing any Randox contract”, but on 9 April last year the MP had a telephone meeting with the firm and Lord Bethell, then a health minister, about Covid testing.

On Saturday, Labour’s deputy leader, Angela Rayner, said she had written to Case demanding “the publication of all correspondence and details of all meetings between ministers and the businesses that were paying Mr Paterson to lobby on their behalf”.

The Observer has been told that before Paterson resigned as an MP on Thursday, Conservative whips understood that Stone was seriously considering launching an investigation into the former Northern Ireland secretary’s lobbying of ministers since the pandemic began. She is also believed to be considering an inquiry into the controversy over the prime minister’s refurbishment of his Downing Street flat.

Angela Rayner: ‘It is particularly brazen that Randox were awarded £347m of taxpayers’ money.’

Over the coming days, the opposition parties are determined to raise the pressure. Rayner said: “It is particularly brazen that Randox were awarded £347m of taxpayers’ money after already failing to deliver on a previous contract that resulted in the recall of 750,000 unsafe testing kits and care homes being left without regular testing. Ministers need to set out how they will claw back taxpayers’ money that was wasted on duff PPE and failed testing contracts.”

The SNP leader, Ian Blackford, said he had also written to the cabinet secretary demanding “full transparency about Paterson’s lobbying, particularly relating to Covid-19 contracts involving huge sums of money”, while the Lib Dem chief whip, Wendy Chamberlain, called for “a full and thorough investigation by the standards commissioner” into the links between Paterson, Randox and the former health secretary, Matt Hancock.

The Lib Dems have secured a three-hour debate on the issue on Monday in which the way Covid contracts were awarded will be central.

The latest Opinium poll suggests the sleaze rows are hitting the Conservatives. The Tory lead has fallen to just one point, from five points a week ago, while Johnson’s personal rating has dropped to -20 from -16 last week, passing a previous low of -18 recorded a month ago.

There has also been a significant shift in views of who would make the best prime minister. An 11-point lead for Johnson last week has shrunk to just two points. Johnson is regarded as the best candidate by 28% of voters, down five, with the Labour leader, Keir Starmer, on 26%, up four.

Sir John Major: ‘The government, with their over-large majority, tend to treat parliament with contempt.’

Speaking on BBC Radio 4’s Today programme, Major was scathing about Johnson’s government, the damage it was doing to the UK’s reputation, and the way it treated parliament. Referring to efforts to block Paterson’s suspension, Major said: “I think the way the government handled that was shameful, wrong and unworthy of this or indeed any government. It also had the effect of trashing the reputation of parliament.”

He added: “There is a general whiff of ‘we are the masters now’ about their behaviour. I’m afraid that the government, with their over-large majority, do tend to treat parliament with contempt. And if that continues, it will end badly.”

Writing in the Observer, the shadow justice secretary, David Lammy, also takes aim at the business secretary over his suggestions last week that Stone should consider her position because of the way she conducted her inquiry into Paterson’s lobbying activities prior to February last year.

“Kwasi Kwarteng’s attempt to bully Kathryn Stone out of her job was yet another breach of the ministerial code, and the latest example of the Tories’ slide into corruption and moral bankruptcy. Boris Johnson has already had multiple run-ins with the standards commissioner. It is clear he simply wants revenge and impunity from the rule of law,” he wrote.

“This level of flagrant norm-shattering and proud lawbreaking demands more than tut-tutting. That’s why we need the adviser on ministerial interests to launch an inquiry into Mr Kwarteng’s threats. And it is why Labour has urged the standards commissioner to open an investigation into the prime minister over the financing of the refurbishment of his Downing Street flat.”