Theresa May has toughened her stance on refusing to rule out a no-deal Brexit, telling Jeremy Corbyn it is an “impossible condition” that she cannot meet.

The Labour leader is boycotting talks with the prime minister until she drops threats to crash the UK out of the EU – and has told his MPs to do the same.

But, in a letter to Mr Corbyn, Ms May has said: “That is an impossible condition because it is not within the government’s power to rule out no-deal.”

She argued that the default position, under the Article 50 process, was that the UK “will leave the EU without a deal on 29 March”.

Ms May wrote: “There are two ways to avoid no-deal: either to vote for a deal, in particular a withdrawal agreement that has been agreed with the EU, or to revoke Article 50 and overturn the referendum result.”

And she challenged Mr Corbyn to make his choice, adding: “I believe it would be wrong to overturn the referendum result.”

The letter came as it was revealed that No 10 has drawn up a paper setting out the potential timetable for a Final Say referendum on Brexit – arguing it would take an astonishing 14 months.

The document’s existence emerged after Ms May referred to it in meetings with other political leaders, as she seeks a consensus on how to solve the crisis.

But Dominic Grieve, a Tory supporter of a fresh referendum, ridiculed the idea as “simply wrong”, adding: “It could be carried out swiftly and within a limited time extension of Article 50.”

Ms May's no-deal claim comes despite both foreign secretary Jeremy Hunt and chancellor Philip Hammond suggesting MPs could – in reality, whatever the legalities – block a crash-out.

In a leaked transcript of a conversation with business leaders this week, Mr Hammond said: “It is clear to me there is a large majority in the Commons that is opposed to no deal in any circumstances.”

In her letter to Mr Corbyn, Ms May also appeared to dismiss the aims of a cross-party campaign, coming to a head next week, to put parliament in charge of the process.

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Put forward by senior Tory Nick Boles, it would compel the government to seek an extension of Article 50, until the end of 2019, if the Commons cannot pass a Brexit deal within a few weeks.

But Ms May wrote: “I do not believe that the EU would agree to extend our membership simply to allow further debate on Brexit in the UK.”

Previously, the prime minister had not ruled out extending membership.

In the letter, she again urged Mr Corbyn to rethink his refusal, in order to “put forward your own proposals”, adding: “I would be happy to discuss them with you.”

She then had a possible dig at Mr Corbyn’s willingness in the past to speak to groups such as Hamas without preconditions, writing: “You have always believed in the importance of dialogue in politics.”