WESTMINSTER is heading for a “constitutional logjam” amid concerns there is not a majority for any option to either leave the European Union or force a fresh vote on staying in the bloc.

Prime Minister Theresa May’s draft withdrawal agreement is widely expected to be rejected by MPs this week, but none of the alternatives appear to have enough support to pass.

These include a fresh referendum on exiting the EU, renegotiating the current deal, producing a radically different alternative, or crashing out with no agreement.

May is heading for defeat after dozens of Tory MPs, as well as the Democratic Unionist Party (DUP), promised to vote down the draft deal she struck with EU negotiators.

The key flashpoint is the ‘backstop’ arrangement which would be triggered in the event of a free trade deal not being struck between the UK and EU.

Critics believe the UK could be trapped into a one-sided relationship with the EU from which it could not withdraw unilaterally.

Defeat could result in May’s critics on the Tory backbenches, or Jeremy Corbyn’s Labour, forcing a vote of confidence in her leadership.

A senior Tory source told the Herald on Sunday that “constitutional logjam” was the most likely outcome of Tuesday’s vote, if it goes ahead.

The insider predicted May would go back to the EU to change the wording of the agreement and use a revised document as the basis for a second Commons vote, but added that such an eventuality would still not be enough.

Labour’s preferred option - a general election triggered by a no-confidence vote - in considered unlikely as Tory MPs and the DUP, which props up the Government, are highly unlikely not vote for an early poll.

Another referendum is backed by the Lib Dems, the SNP and, to a lesser extent, Labour, but supporters do not believe a majority of MPs are currently in favour.

However, an SNP MP said, of all the options, a so-called People’ Vote has the “momentum” and would attract more support amongst parliamentarians if May loses.

He also said delaying the UK’s exit from the EU, which is scheduled to take place in the spring, could attract majority support if no solution looks like being found.

Meanwhile Work and Pensions Secretary Amber Rudd has voiced support for a Norway-style model as an alternative if the draft withdrawal agreement is thrown out by MPs.

Rudd, who is is the first Cabinet minister to publicly discuss the merits of a "Plan B", also said she would not rule out a second referendum and suggested she would back remaining in the EU if one was held.

She said she supported the PM's withdrawal agreement and took aim at "indulgent" critics of the deal, including male MPs who "think they are standing on principles but they're not getting things done".

She revealed that, should the deal be voted down, she would prefer a so-called Norway Plus model for Brexit that would involve staying part of the European Economic Area.

The former home secretary said the alternative "seems plausible not just in terms of the country but in terms of where the MPs are," but conceded that "nobody knows if it can be done".

However, Norway Plus would mean the continuation of the free movement of people from the EU, the ending of which is a red line issue for the Prime Minister.

On a second referendum, Rudd said she "can't understand the hysteria around a People's Vote if you believe in what you are trying to propose", but said she would "much rather do the withdrawal agreement”.

In another development, former Tory leader Lord Howard said May would have "difficult decisions to make about her future and about the future of our country" if she loses the Brexit vote.

Lord Howard said discussions with the EU should be intensified to prepare for Brexit without a full withdrawal agreement.

"We should seek to put in place some ad hoc, temporary arrangements with the agreement of the European Union which would minimise and, indeed, perhaps even eliminate any disruption at the border on March 30 next year," he told the BBC.

"We should also undertake that we would unilaterally, for the period of 12 months after March 29, allow any goods and services in from the European Union without any tariffs or tariff barriers or obstacles in any way - hope that they will reciprocate but do it even if they don't - and use that 12-month period to negotiate a free-trade agreement along the style of Canada-plus."

It came as Labour shadow chancellor John McDonnell said there would be a “vacuum” if, as he hopes, Parliament votes down the May. deal.

Speaking to journalists in a school in Airdrie yesterday, he was coy about Labour’s tactics in the event of the Prime Minister losing the vote, saying that his party would make judgements “day by day”.