Boris Johnson could be forced to delay Brexit as MPs fearing a no-deal ambush bid to scrap a crunch vote on his deal. 

The prime minister will ask the Commons to back his deal as parliament sits on a Saturday for the first time in 37 years tomorrow.

The vote is happening at the weekend as the Benn Act, which forces the PM to seek a Brexit extension if the UK still faces no-deal, kicks in at 11pm Saturday. 

But the alliance of Labour, SNP, Lib Dems and former Tories are worried that – if Johnson can cobble together a majority for his new agreement – Tory Eurosceptics could game a no-deal Brexit by later rejecting legislation that would put the deal into law before October 31. 

So, as an insurance policy, Sir Oliver Letwin, the ex-Tory who spearheaded the bid for parliament to seize control of the order paper, has tabled an amendment to the government’s motion that says MPs will withhold backing for the deal until the supporting legislation is safely passed. 

If selected by Speaker John Bercow and a majority of MPs back the plan, it will automatically trigger the anti-no-deal Benn Act, and Johnson may be forced to seek an extension to the Halloween Brexit deadline from Brussels. 

A delay to the crunch vote does not mean that the UK cannot leave the EU on October 31, but it means the government has just days to push through legislation. 

But it comes as European leaders cast doubt over whether they would back a Brexit delay, with France’s Emmanuel Macron and Ireland’s Leo Varadkar both warning it could be rejected.

Varadkar said: “The position that we have agreed as the European Council is that as things stand there is no request from the UK for an extension, if for some reason a request came, then Donald Tusk – president of the European Council – would consult individually with all the European leaders to see if we would agree to such a request,” he said.

“But bear in mind that request would have to be agreed unanimously by all 27 leaders, so I don’t think MPs voting tomorrow should make the assumption there would be unanimity for an extension. But our point of view has always been that we would be open to it, but it would be a mistake to assume that it’s a guarantee, given that it requires unanimity by all 27 member states.”