Last month while being grilled by a committee of MPs, Brexit Secretary David Davis said anyone who went into a negotiation trumpeting their red lines was an "idiot".

Unfortunately for Mr Davis, that is precisely what his boss Theresa May has done.

At the core of the Brexit vision set out a year ago in the Prime Minister's Lancaster House speech was the fact the UK was leaving the single market and the customs union.

In the last few weeks, Mrs May restated that Lancaster House fact - kiboshing suggestions she was contemplating a post-Brexit customs union of any kind.

It is a clear red line, and one of her own making.


But there are growing questions about whether there is a majority in the House of Commons for it.

Cross-party amendments to the Trade Bill have been tabled by some rebel MPs from Mrs May's own backbenches, which - if passed - would require the Government to enter into a customs union with the EU.

Backing from the Liberal Democrats and the SNP for such proposals was always a given, but Labour's shift in stance to now support a customs union has upped the ante.

Tory rebels like Anna Soubry and Ken Clarke have already inflicted one defeat on the Government over Brexit and, if they are able to garner similar numbers, they could do so again.

But this time with far more significant repercussions.

:: What you need to know about customs union

Image: Theresa May's position is likely untenable if she reneges on her Brexit red line

The nervousness in Government is obvious. Senior figures are understood to be considering the brinkmanship of recasting a vote on the amendment as a vote of confidence in Mrs May's leadership.

Little wonder then, that Labour now sense this is the moment to exploit that fear.

As a political play, Jeremy Corbyn's expected announcement that Labour will back remaining in a customs union with the EU is motivated far more by the prospect of collapsing the Government and forcing a general election than anything to do with frictionless trade.

It is not so much an evolution of policy as an escalation of Labour's efforts to bring Mrs May down.

That is what shadow Brexit secretary Sir Kier Starmer means when he says "crunch time" is approaching for the Prime Minister.

He is not talking about questions on policy specifics, he is talking about whether Mrs May can avoid defeat at the hands of rebels in her own party if Labour swing in behind them.

Image: Anna Soubry has already helped inflict one defeat on the Government over Brexit

The first defensive step taken by the Government has been to buy time by pushing discussion of the Trade Bill back by a few months, giving the Tory whips space to exert pressure on potential rebels to back down.

International Trade Secretary Liam Fox urged those Tory rebels on Sunday to keep an "open mind" ahead of an upcoming speech on Brexit the Prime Minister will deliver in the North East on Friday.

Dr Fox said he was confident that once pro-Remain Conservatives hear Mrs May set out the Brexit position agreed between senior Cabinet members at Chequers last week, many of their concerns would be put to rest.

Yet, given even some of those present at the Chequers meeting are understood to have doubts about how resilient the delicately-balanced Cabinet agreement will be - once it's set out at the Brussels negotiating table - it seems unlikely Tory rebels will be reassured in any lasting sense.

But, even if they were and they chose to withdraw the amendment, to some degree the damage is already done.

Image: There are growing questions as to whether the PM has a majority in the House of Commons for her Brexit plan

They have already demonstrated enough disquiet to prompt Labour to shift its Brexit position in order to capitalise on the potential for a House of Commons majority that might defeat the Government.

This is likely to be ongoing pressure point for the Prime Minister, and yet it is hard to see what Mrs May can do to reduce that pressure.

The Prime Minister's commitment to leave the customs union is perhaps the clearest thing she has ever said on Brexit.

It is the reddest of red lines and Mrs May's position is likely untenable if she reneges on it.

And yet, given the parliamentary arithmetic, there are real questions over whether the Prime Minister can deliver on it.

Mr Davis' musings to the select committee on the wisdom of setting red lines in negotiations was delivered in the abstract.

But, in the context of the parliamentary battles ahead, it may have been unwittingly prescient.