Labour says it will join Conservatives to block 'no deal' Brexit

McDonnell's comments came as a cross-party group of MPs including several former Conservative ministers revealed plans that would give parliament the ability to veto, or prevent by other legal means, a "bad deal" or "no deal"outcome, using amendments to the forthcoming EU withdrawal bill.

McDonnell said Labour was not prepared to consider the prospect of leaving the EU without a negotiated settlement. "I'm not willing to countenance that. I don't think there is a majority in parliament for no deal," he told the BBC's Andrew Marr Show.

"I think there are enough sensible people in the House of Commons to say, 'This cannot happen, we cannot damage our country in this way.'"

Asked if Labour would work with Conservative MPs who were also concerned about leaving with no deal, McDonnell said: "There are discussions going right the way across the House.

"I worry for our country as a result of what is happening within the Conservative party. This is not about negotiating with Europe, it's about fighting amongst themselves and if they can't negotiate they should get out of the way and let us do it."

However, the transport secretary, Chris Grayling, told the same programme that parliament would have no mechanism to keep the UK in the EU for longer than 30 March 2019, which is when Britain's membership of the bloc will automatically expire if no deal is reached. "Parliament has already voted for leaving the European Union overwhelmingly," he said.

Theresa May has made it clear that at the end of negotiations MPs will have only two options.