Theresa May has explicitly ruled out a no-deal Brexit for the first time, in what Conservative MPs are interpreting as a last-ditch attempt to win support ahead of a fourth vote on her Brexit deal.

The Prime Minister warns this evening: “Because Parliament has made clear it will stop the UK leaving without a deal, we now have a stark choice: leave the European Union with a deal or do not leave at all.

“My answer to that is clear: we must deliver Brexit and to do so we must agree a deal.”

Tonight's statement makes explicit what was heavily implied in her Downing Street speech of Tuesday, in which she pledged to seek a long extension and forge a cross-party compromise with Labour rather than pursue no-deal.

The inevitable logic of that position was that the Prime Minister would never countenance leaving without an agreement, though she stopped short of saying so and instead stressed: “Leaving with a deal is best solution.”

Tonight, however, the stark binary choice Downing Street has always believed would ultimately face Conservative MPs has been articulated in black and white under May's name for the first time: vote for the Prime Minister's deal, or else lose Brexit.

May adds: “The longer this takes, the greater the risk of the UK never leaving at all. It would mean letting the Brexit the British people voted for slip through our fingers. I will not stand for that. It is essential we deliver what people voted for and to do that we need to get a deal over the line.”

Conservative MPs believe the Prime Minister is making a final attempt to spook recalcitrant backbenchers into voting through the withdrawal agreement. “You don't have to be Brain of Britain to work that out,” says one former rebel who voted with the government at the third time of asking.

May is also seeking to increase the pressure on Jeremy Corbyn, whose team declared yesterday that negotiations over a compromise could not continue unless May offered meaningful concessions on her red lines.

In a thinly-veiled threat to both Corbyn and her own party, May warned that a cross-party compromise would be “the only way to deliver Brexit” if she could not count on Tory and DUP votes. Recent Conservative attack ads have focused heavily on the Corbyn's refusal to back the withdrawal agreement, a line some Labour MPs fear would form the central plank of a snap election campaign.

“The referendum was not fought along party lines and people I speak to on the doorstep tell me they expect their politicians to work together when the national interest demands it,” she said. “The fact is that on Brexit there are areas where the two main parties agree: we both want to end free movement, we both want to leave with a good deal, and we both want to protect jobs. That is the basis for a compromise that can win a majority in Parliament and winning that majority is the only way to deliver Brexit.”

Privately, May loyalists have launched a drive to drum up support for the prime minister's attempt to broker a compromise with Corbyn – a move that Eurosceptic holdouts also suspect is a ruse to bounce them into folding.

Nominally led by Prisons Minister Rory Stewart – the most enthusiastic and reliable salesperson for May's deal on the government payroll – Conservative MPs have been urged to add their names to a letter intended to “counter-balance some of the negative stories” about talks with Labour.

Full text of the message circulated among Tory MPs is below.

Many apologies for disturbing – but it does seem – from my perspective anyway – as though it could be hugely helpful for the PM, if we could express some support for what she is trying to do at the moment and counter-balance some of the negative stories of division. A group of us are, therefore, getting a letter together expressing our support for her and the government negotiations. The text of the letter would be kept private. And so would the list of signatories. If you would be willing to support that would be wonderful. Your name would not be quoted. Text attached. Fully understand, of course, if the answer is no but would be very grateful for a “yes” or “no” as soon as possible. BEGINS Dear Prime Minister, We are writing to express our support for the Cabinet’s decision to open Brexit discussions with the Opposition in the spirit of compromise. This is not a path that any of us wanted or expected to have to take. Equally, we recognise that doing so will have been as uncomfortable for you as it has been for us. But it is high time that we secured a Brexit deal, and got on with implementing it. 17.4 million British people voted to leave the European Union in the largest democratic exercise in our history. All conservative and Labour MPs promised to implement the result. And we need to honour that promise. We regret that the deal you and others negotiated with the European Union has not been approved by Parliament. It was not a perfect deal – no compromise ever could be – but it was a good deal. It delivered on the verdict of the British people, whilst enabling the orderly departure which was promised and which protected the jobs and livelihoods of our constituents. We disagree profoundly with Corbyn’s Labour party and its policies, so engaging with him in normal circumstances would be unthinkable. But these are not normal circumstances. We are disappointed that a number of colleagues remain unable to support the Cabinet’s Brexit deal, which has now been voted down three times. Faced with such a position, we recognise (however reluctantly) that reaching out to the Opposition has become arithmetically unavoidable. It is high time that we finalised a Brexit deal, and left the European Union, rapidly and in an orderly, thoughtful fashion, so that we can begin to take the steps to make a success of Brexit, and all our pressing domestic priorities. ENDS

Tory MPs who remain opposed to the prime minister's deal showed no sign of succumbing to Downing Street's overtures last night. Steve Baker, the deputy chair of the European Research Group, told the New Statesman: “At a time when Conservative members and activists are recoiling in horror at the idea of collaborating with Jeremy Corbyn, the No 10 effort to rehabilitate this awful deal and worse strategy to drive it through seems wholly forlorn.”