Hardline Brexiteers were today digging in to inflict a fourth defeat on Theresa May's Brexit plans. They believe the Prime Minister will fail if she attempts to twist their arms by pitting her proposals against plans for a customs union.

Veteran backbencher Julian Lewis told the Standard:

"If it comes back a fourth time, I can't conceive of circumstances where those people who have already rejected it three times would change their mind."

He stressed that at least 15 Brexiteers still opposing Mrs May's Brexit blueprint had backed the "clear moral course" that 

"it is not our fault that we are confronted by two unacceptable choices, but it will be our fault if we cast a positive vote in favour of either for fear of the other".

MPs were tonight voting on alternatives to the Government's Brexit proposals, with one option being pushed by Father of the House Ken Clarke, for a customs union, appearing likely to emerge as among those with the strongest backing. However, Brexiteers are prepared to vote against Mrs May's plan even if the possibility of a custom union is used to try to force them to fold, with some believing the revolt could grow.

Former Welsh secretary David Jones said:

"You are not going to change your stance because there is another equally bad choice. I don't think anybody who has voted against the [withdrawal] agreement is going to fall for that."

Former environment secretary Owen Paterson said:

"I will vote against both." 

Former Welsh secretary John Redwood doubted whether Speaker John Bercow would allow a fourth "meaningful vote" (MV4) but added: 

"We are not going to change our mind."

Wellingborough MP Peter Bone stressed: 

"I won't vote for Mrs May's deal by being threatened with anything." 

If there was a "genuine" change to the Prime Minister's plan, he would consider looking at it again.

"But otherwise I will vote the same way I have done before," 

he added. Basildon and Billericay MP John Baron said: 

"Will vote against MV4 unless it is amended to allow the UK to leave the backstop without EU permission."

Tewkesbury MP Laurence Robertson said: 

"Unless it [the withdrawal agreement] changes significantly I'll have to continue to vote against it."

Yeovil MP Marcus Fysh emphasised: 

"PM's deal is a customs union. EU will just run down the clock and insist on that [the backstop]."

The Democratic Unionist Party is also standing firm, with its Brexit spokesman Sammy Wilson telling the BBC: 

"[Should] she bring it back 1,000 times, we will vote against it. The implications for Northern Ireland are far, far too serious."

Mrs May would appear to need more Labour MPs than the five who backed her last week to get her deal through the Commons.