Theresa May faces a tough grilling from senior MPs over her controversial Brexit agenda, but has been boosted by strong backing from a prominent Cabinet Leave supporter.

The Prime Minister is expected to receive intensive questioning as she appears before the Commons Liaison Committee, which is made up of leading parliamentarians, on Thursday.

The appearance comes as Labour threw down the gauntlet over Mrs May’s EU withdrawal agreement by tabling an amendment opposing the deal ahead of a Commons showdown in 12 days’ time.

However, the PM gained support when prominent Brexiteer and Commons Leader Andrea Leadsom backed Mrs May’s stance.

Mrs Leadsom, who had been named as one of the “pizza club” of pro-Leave Cabinet ministers who held informal meetings to discuss their response to withdrawal plans, backed Mrs May’s deal, the Daily Mail reported.

In a letter to constituents Mrs Leadsom said it had been a “challenging journey”, but the PM’s plan was the only deal on the table and meant the UK would quit the bloc next March, the newspaper said.

The intervention came as the Government confirmed MPs will debate the Brexit deal eight hours a day for five days leading up to a crunch vote on December 11.

MPs will be allowed to vote on six amendments to the Government motion backing the deal during the Commons showdown.

Commons Speaker John Bercow will decide which amendments get to be debated and decided upon by MPs ahead of the so-called meaningful vote on Government proposals.

Labour leader Jeremy Corbyn, who is appearing on ITV’s This Morning on Thursday to push his case, said the party could not back Mrs May’s plan as it failed to ensure participation in a “strong” single market and customs union.

The move came as Labour signalled a new referendum would be inevitable if Mrs May’s plans are voted down.

The comments followed a warning from the Bank of England that without an exit deal the UK could be tipped into a recession worse than the financial crash, with an 8% cut in GDP, unemployment surging by as much as 7.5% and house prices falling by almost one-third.

Also a cross-Government analysis found the UK economy would be 9.3% smaller after 15 years if Britain leaves without a deal and falls back on World Trade Organisation rules, compared with remaining in the EU.

While the UK economy would continue to grow after withdrawal, Britain would be worse off under any Brexit scenario than if it stayed in the EU, the Government paper found.

There could be further stark warnings on Thursday, when the Commons Treasury Committee publishes the Financial Conduct Authority’s analysis of the Brexit deal and political declaration.

But arch-Brexiteer Jacob Rees Mogg was dismissive of all the claims, telling the Daily Telegraph: “This is Project Hysteria.

“Before the referendum we were threatened with a plague of frogs.

“Now they warn of the death of the first born.

“The Bank of England has gone from being discredited to being hysterical.”

Pro-Europe Tories hit back with former minister Jo Johnson urging party colleagues to vote down Mrs May’s deal.

The ex-minister, who is brother to leading Brexiteer Boris Johnson, is using a speech on Thursday to say that if the Conservatives do not change course they face a bigger electoral defeat than 1997.

And security minister Ben Wallace will say that a no-deal Brexit would leave both the EU and UK at greater risk.

He will say on Thursday that leaving the bloc without an agreement would have a “real impact” on authorities’ ability to protect the public.