The assessment, which is expected to be published next week, will compare the long-term impacts on the UK’s economy of the Prime Minister’s Brexit plan, no deal and the UK remaining in the EU.
The move is thought to be an attempt to boost support for Mrs May’s deal ahead of the Commons vote, following repeated threats by Tory rebels to vote against the agreement.
Treasury Minister Robert Jenrick told MPs on Monday the Government would now release a comparative analysis of the economic impact if Britain was to remain in the EU rather than leave in March 2019.
Previously, the Government had only undertaken to publish its assessment of the impact of leaving with a deal compared to a no-deal Brexit.
But Mrs May’s Government has promised to publish a range of analyses to help lawmakers decide whether to back her plan for a close trading relationship with the EU after Brexit in a vote in Parliament most likely to happen early next month.
Some MPs say publishing a comparison of the economic impact of Mrs May’s deal against that of staying in the EU will press the case that Britain should drop its Brexit plans.
Mrs May is travelling to Brussels today for crunch talks with the EU Commission President Jean-CLaude Juncker to finalise her Brexit deal.
And the Prime Minister’s allies have called on Tory MPs to back Mrs May or risk the UK never leaving the EU at all.
Speaking to BBC Radio 5 Live’s Emma Barnett Show, Chief Secretary to the Treasury Liz Truss said: “If my colleagues in Parliament don’t vote for this then we’re in grave danger of not leaving at all.
“I am very very worried about what the alternative looks like, because I’m a democrat. I believe that people voted in good faith to leave the EU and we need to deliver that.”
Ms Truss added she was “confident we’ve got a good deal to send before Parliament”.
The shock warning was echoed by work and Pensions Secretary Amber Rudd, who returned to Cabinet this week following Esther McVey’s resignation in protest of the Prime Minister’s Brexit deal.
The new Work and Pensions Secretary told BBC Radio 4’s Today programme: “If it doesn’t get through, anything could happen.
“The Brexiteers may lose their Brexit.”
But Ms Rudd ruled out the possibility of a no-deal Brexit.
She said: “It is my view that the House of Commons will stop no-deal.
“There isn’t a majority in the House of Commons to allow that to take place.”
Ms Rudd insisted she expects Mrs May’s deal to pass through the Commons.
She said: “I think people will take a careful look over the abyss – MPs of all parties – and consider whether they think it is in the best interests of the whole country.
“I think the likelihood is, despite what people say, that the Withdrawal Agreement will get through.”