Theresa May wants the EU to sign a declaration on the Brexit backstop to reassure angry MPs in Westminster.

The prime minister is set to ask Brussels to make a strong commitment to avoiding the backstop coming into effect.

On Tuesday the government was accused of planning this statement, or addendum, weeks before the meaningful vote debate got underway.

A Cabinet source told Business Insider that May's government discussed an addendum last month.

The prime minister allegedly promised to add an addendum to the Brexit deal to stop Cabinet Brexiteers resigning.

A spokesperson for the prime minister said this was "not true."

LONDON — Theresa May promised her Cabinet a new text on the Brexit backstop to accompany the Withdrawal Agreement weeks before the meaningful vote debate got underway, a Cabinet source has told Business Insider.

The prime minister is currently in talks with EU leaders in an attempt to secure what she described as "additional reassurance on the backstop," expected to come in the form of an addendum to the Brexit withdrawal deal, confirming that the EU does not want to use the backstop and is committed to finding alternative arrangements.

The backstop is controversial in Westminster because it could keep the UK in a customs union with the EU indefinitely while Northern Ireland would stay in parts of the single market, creating new border checks with Great Britain.

The move to seek an addendum to accompany the Withdrawal Agreement comes after May pulled the House of Commons vote on the deal on Monday, saying that she had listened to MPs' concerns about the backstop.

However, a Cabinet source told BI that Downing Street officials had in fact instructed civil servants to work on a draft addendum soon after Dominic Raab quit as Brexit Secretary last month, weeks before the meaningful vote debate.

Pro-Brexit ministers told May they'd stay in government if "she got changes to the political declaration to include putting other things on the table than the backstop, and the addendum to give us ways out of the backstop," one Cabinet source told BI.

"They were then drafted by civil servants after [Dominic] Raab left," they added.

This was in anticipation of a backlash from MPs and as a way of persuading pro-Brexit members of Cabinet — like Penny Mordaunt, Chris Grayling and Andrea Leadsom — to not resign from government.

A spokesperson for the prime minister described the claim as "not true" on Tuesday afternoon.

Labour Mp Stephen Doughty. YouTube/BBC News

On Tuesday Labour MP Stephen Doughty accused the government of drafting a new text to accompany the Brexit deal "weeks before" the parliamentary debate on the meaningful vote started last week.

"Can he tell me, has any member of the Cabinet seen or discussed a draft of addendum that the prime minister is seeking at any point over the last few weeks before the prime minister decided to postpone the debate? Has any member of the cabinet seen or discussed it?" Doughty asked government minister David Lidington.

Lidington, May's de facto deputy prime minister, did not deny Doughty's accusation, telling him: "He will certainly understand that I am not going to discuss the cabinet discussions that take place within Cabinet meetings."

Doughty, a supporter of the People's Vote campaign, said the government is "treating Parliament and the people of this country with contempt, hiding the truth from them and attempting to force their Brexit withdrawal agreement on the country through backroom deals and behind-closed-doors horsetrading."