When MPs begin to have meaningful votes on the amendments to the Brexit bill Theresa May faces real peril.

At the weekend Downing Street sources went as far as to label next month's planned binding vote on the customs union as one of confidence in the prime minister.

Asked what they thought of this notion this week several Norfolk MPs played it down. But off the record some admit that if the government can't get the EU (Withdrawal) Bill through there is every chance Mrs May's days in Number 10 will be numbered.

Yesterday's debate on the customs union proved the clear split among MPs. The vote is going to be tight.

The Conservative whips office has now gone into overdrive. They will be using all their guile and, perhaps, some of the dark arts their role has become synonymous with.

Mrs May promised to deliver the type of Brexit she believed the British public had voted for – that meant quitting the customs union and the single market. Basically, going it alone.

What type of Brexit – agree with it or not – Britain achieves rests in the hands of the whips.

The term whip has its origins in hunting. The whipper-in made sure the hounds stayed in the pack and did not wander off or lag behind. And that is the main role of the whips in parliament.

But they do so much more as well.

The whips are the prime ministers eyes and ears. It is their job to know when MPs are wavering from the party line – and to know if they are up to no good.

Last year questions were raised over methods used by the whips. It is no secret that if the whips get a whisper of some bad behaviour they will use it against MPs if their support for the government falters.

Former chief whip and current defence secretary Gavin Williamson played the role of government enforcer with gusto. He was – and remains – the real-life Francis Urquhart.

He even keeps a tarantula called Cronus – named after the Greek god who came to power by castrating his own father – on his desk in an apparent bid to spook wayward MPs. I imagine it works.

In his conference speech last autumn he said in a slightly menacing tones: 'I don't very much believe in the stick, but it's amazing what can be achieved with a sharpened carrot.'

As those Lords' amendments start returning to the Commons you can almost hear the root vegetables being readied for action.

Obviously when the amendments are voted on MPs will be whipped – told which way to vote by their parties. Some will rebel – Ken Clarke, Anna Soubry – but others will vote in a way they perhaps would not have otherwise. The reason for that is to protect the government.

But Brexit is a decision that will impact generations to come. Its shadow will stretch way beyond Mrs May time in office.

Ideally MPs would be offered a free vote on all Brexit matters. That would allow Labour leavers and Tory remainers to vote in the way they truly believe will bring about the most successful Brexit for the country.

But Brexit has already claimed too many scalps for that. The people might have made the decision by voting narrowly to leave but now MPs are playing politics with it.

Jeremy Corbyn, Boris Johnson, Jacob Rees-Mogg – they are queuing up waiting for Brexit to topple Mrs May. That is why Mr Johnson keeps laying down Brexit red lines, Mr Rees-Mogg has become the leader of the Tory backbenchers and Mr Corbyn is being fairly non-committal on leaving the EU.

Nothing has been easy for Mrs May since she took office. There have been numerous moments when her MPs have privately said she would soon be asked to step aside. She has always survived.

But now she needs her whips more than ever – and they will need more than sticks and sharpened carrots.