London, October 14: Indian-origin MP Priti Patel, one of the most outspoken British politicians in favour of Brexit, joined fellow Brexiteers on Sunday to mount a further challenge to Prime Minister Theresa May's plans for a future trading arrangement with the European Union (EU).

Patel is among 63 Conservative Party MPs to sign a letter attacking the UK government over its Brexit forecasts and blaming May's team of leaking negative financial forecasts of the impact of Britain's exit from the economic bloc.

"Adopting a constructive and transparent approach would be in the national interest," says the letter, organised by the Economists for Free Trade group of Eurosceptic economists and former Brexit minister Steve Baker. Former UK Foreign Secretary Boris Johnson calls Theresa May's Brexit Plan a 'Suicide Vest'.

"The general public is battered and bewildered by conflicting predictions of the future path of the economy following Brexit, fuelling a growing suspicion that Whitehall is engaging in what is apparently known internally as 'policy-based evidence-making'," it notes.

The show-of-unity letter, addressed to UK Chancellor Philip Hammond, comes as one of its signatories – former Brexit Secretary David Davis – openly called for a Cabinet uprising against May's Brexit strategy ahead of a crucial summit with the EU in Brussels scheduled for next week.

Writing in The Sunday Times, the former minister said the government's negotiating strategy had "fundamental flaws", arising from the "unwise decision in December to accept the EU's language on dealing with the Northern Ireland border".

The British PM has suggested a temporary customs arrangement for the whole of the UK to remain in the EU Customs Union while the complicated Irish border issue is resolved.

However, Brexiteers like Patel and Davis suspect this could turn into a permanent situation, restricting Britain's freedom to strike future trade deals with other countries.

"This is one of the most fundamental decisions that government has taken in modern times. It is time for the Cabinet to exert their collective authority. This week the authority of our constitution is on the line," writes Davis, who stepped down from the UK Cabinet in July due to his opposition to May's Brexit strategy agreed by the rest of the Cabinet at her country residence of Chequers.

A few days later, he was joined by another vocal Eurosceptic minister, Boris Johnson, who stepped down as Foreign Secretary and has since issued a series of direct attacks on the British PM's plans over the issue of Brexit.

In his newspaper article, David Davis also accuses the Prime Minister's aides of "incipient panic" in the face of Brussels' demands and wrongly trying to cut out the Cabinet from the key decisions.

May now faces an onslaught from at least nine ministers wanting her to change direction when the Cabinet meets on Tuesday — with credible threats to resign from at least four. They are demanding a date be set to leave the Customs Union or a break clause, to be triggered in London, not Brussels.

Andrea Leadsom, the Leader of the Commons, is said to be "considering her position."

Scottish secretary David Mundell and Ruth Davidson, leader of the Scottish Tories, have also made clear they would resign if Northern Ireland faces new controls that separate it from the rest of the UK — because that would fuel the case for Scottish independence. In a further blow for the British PM, many anti-Brexit MPs who had voted to remain in the EU now plan to vote against her deal.

The Democratic Unionist Party (DUP), which supports the government as part of a pact to make up for the Tory government's lack of majority in the House of Commons, has warned May not to sign up to any deal that would see Northern Ireland remaining in the EU single market while the rest of the UK left.

Britain is set to formally leave the EU on March 29 next year after a 2016 referendum in favour of Brexit. Both sides are working towards a mid-November deadline for agreeing their so-called "divorce" arrangement, before they can formalise a future trading relationship by January next year.

The issue of the border between Northern Ireland and the Republic of Ireland, which will become the UK's border with the EU, is one of the last remaining obstacles to achieving a divorce deal with Brussels.

A pre-agreed transition period is set to end in December 2020, following which, unless there is an agreement in place, the UK would exit the economic bloc without a deal – a scenario that has been painted as dire by experts.