The Tories have been forced to insist that they won't stitch up a post-election deal with the DUP, Nigel Farage's Reform UK or even the SNP to cling onto power.
The Prime Minister's press secretary repeatedly refused to say whether the Tories would enter a coalition or sign a less formal confidence and supply agreement if they fail to win a majority.
It came moments after Mr Sunak goaded Keir Starmer at PMQs for being "busy plotting coalitions".
But hours later, the Conservatives released a statement saying: "We will not be doing a deal with any other party."
The Tories have sought to resurrect David Cameron's 2015 attack over a "coalition of chaos" between Ed Miliband and Nicola Sturgeon
Keir Starmer refused to rule out striking a deal with the Liberal Democrats to prop up a Labour government but insisted his party was gunning for an outright majority.
Mr Sunak is now facing questions about the desperate measures the Tories might take to cling onto power.
The Conservatives were hammered at the polls in last week's local elections, losing more than 1,000 councillors as voters abandoned Mr Sunak's party.
Projections of national vote share put Labour was on 35%, the Conservatives on 26% and the Lib Dems on 20%, with other parties on 19% - which could lead to a hung parliament at the next election.
Asked if they would agree a confidence-and-supply arrangement or a coalition with any party, the PM's press secretary said: "I don't think anyone at this stage is going to speculate on the results of the next election.
"The Prime Minister is fully committed to and focused on delivering his five priorities and that's what we're going to do to get a Conservative majority."
The "anti-woke" Reclaim Party, founded by Laurence Fox, said it was open to pacts with other smaller parties, including Reform UK, which emerged from the ashes of Nigel Farage's Brexit Party.
But Mr Fox, a former actor turned 'anti-woke' crusader, wants to target "problematic" Tory and Labour MPs at the next election.
Mr Fox said: "We're not looking to stand in 650 seats. We're looking to find problematic Tories particularly, or Labour and stand there, stand on our principles where wokery is taken off terribly and cancel culture exists."
Asked about pacts with Reform and other right-wing parties, he said: "We support anybody who is challenging this two cheeks of the same arse Parliament where both parties agree on everything and the people don't agree at all."
Labour urged Mr Sunak to rule out a "grubby, desperate deal" with the SNP.
Shadow Scotland Secretary Ian Murray said: "Rishi Sunak's refusal to rule out a grubby deal with the SNP is a sign of his desperation to cling to power.
"A repeat of the Tory-Nat coalition previously seen in Scotland would wreck the UK, putting the very foundations of our country at risk.
"Under Keir Starmer and Anas Sarwar's leadership, Labour has said repeatedly we wouldn't do any deal with those who want to break up our country."
But SNP Westminster Deputy Leader Mhairi Black hit back, saying: "By talking up the prospect of Tory coalitions, bungling Ian Murray has unwittingly admitted the UK is on course for a hung parliament, contradicting Keir Starmer, and making a spectacular boomerang attack on his own Tory-backing party."
