A group of Labour MPs will refuse to vote to force Theresa May to reveal her Brexit plans – because it would also mean backing her timetable for triggering Article 50.

The Prime Minister’s strategy to split Labour and expose Opposition MPs refusing to “abide by the referendum result” appeared to be working ahead of today’s showdown debate.

Last night, the Labour leadership said it would back a Government amendment even though it “calls on the Government to invoke Article 50 by 31 March 2017”.

Brexit Secretary fails to keep a straight face about 'always intending' to publish Brexit plans

The party is determined to focus on its part of the motion – which commits Ms May to "publishing the Government’s plan for leaving the EU before Article 50 is invoked”.

That was the wording of Labour’s original motion, which the prime minister was forced to accept, faced with a revolt by up to 40 of her own backbenchers.

But four Labour MPs have announced they will defy Jeremy Corbyn by refusing to back the motion, because Ms May has tagged to it a commitment to invoke Article 50.

Former minister David Lammy told The Independent that the government had “conceded ground” by agreeing to publish a plan – but it was nowhere near enough.

He said: “Does this mean that the Government are just going to publish a document saying ‘we will seek the best possible Brexit, and aim for the best possible access to the single market’ and then say ‘there you go: there’s our plan’.

“Will it answer specific points on single market membership? On free movement? On co-operation with our allies on security? On workers’ rights? Environmental protections? Will there be a list of so-called red lines? We don’t know.

“So I cannot vote for a motion today that commits this House to invoking Article 50 in March next year.”

Mr Lammy will be joined by another former minister, Ben Bradshaw, who tweeted: “I will not vote today to invoke Article 50 by March when we still have no idea what sort of #Brexit the Government will pursue.”

Neil Coyle, like Mr Lammy, a London MP, tweeted: “I won't be voting for the Government amendment today. I cannot support triggering Article 50.”

And Mike Gapes tweeted: “I'm told I should be happy that May is now to publish a Brexit plan. But I'm not going to vote for a pig in a poke. What is the plan?”

Ten other Labour MPs have previously backed a motion stating that Article 50 must not be triggered until the Government has set out “the full proposals it intends to submit” to European Commission.

It means there is a potential for a larger revolt, in a vote to be forced by the Liberal Democrats whose leader, Tim Farron, condemned “a parliamentary stitch up”.

Mr Farron said the amended motion failed to “include any meaningful commitment from the Conservative Brexit government to produce the equivalent of a white or green paper”.

It is also possible that some of the Labour rebels will abstain, rather than vote against their party leadership.

Growing frustration that the Government is keeping its own MPs in the dark forced Ms May to back down last night, to avoid an embarrassing defeat.

Labour hailed a “hugely significant climbdown” –demanding that the plan she had agreed to publish be made available “no later than January” next year, to allow for proper scrutiny.