What are party whips and what does a three-line whip mean?

The Tory chief whip’s recent disparaging comments about his own colleagues has once again shone a spotlight on the issue of party discipline at Westminster.

Julian Smith said Conservative MPs who persist in defying Prime Minister Theresa May and the Conservatives' official party line are “the worst example of ill-discipline in cabinet in British political history”.

In a new BBC documentary The Brexit Storm, Smith used a rare public appearance to accuse unruly colleagues of “trying to destabilise” the PM.

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It's not been a good few weeks for the Tory chief whip as “he also appears to have been unable to enforce discipline within his own team”, says the Daily Telegraph.

Last week, a number of whips abstained on the vote to change the date of Brexit in domestic law, “despite being charged with getting fellow Tory MPs to back it themselves”, the paper adds.

This week Nigel Adams quit his position as government whip after May committed to working with Labour leader Jeremy Corbyn to deliver Brexit.

But what is the role of a whip and what punishment can MPs who don't follow the party line expect for their disloyalty?

Who are the whips?

Whips are named after fox-hunting's "whipper-in" - the rider charged with cracking a whip to prevent hounds straying from the pack – and play a crucial role in maintaining party discipline. It is their job to ensure loyalty in key votes and report any signs of unrest to their party leaders. The government's chief whip also usually sits on the Cabinet in the post of Parliamentary Secretary to the Treasury.

While smaller parties have two or three whips, "each of the two main parties will have about 14 MPs appointed to their whips' office", says the BBC.

Every week, each chief whip sends a letter to all its MPs notifying them of the dates of forthcoming votes and reminding them of the party's official stance on the issue in question.

Depending on how important the party leaders consider a particular bill, the whip can be classified as single-line, two-line or three-line (see below).

Getting on the wrong side of a whip can have serious consequences, as former foreign secretary Jack Straw found out during a run-in with a deputy chief whip who "pushed him against a wall and grabbed him by the testicles", the Daily Telegraph reports.

The whips are also in charge of MPs pairing arrangements where MPs are matched with opposition colleagues who cannot vote and abstain in order to maintain fairness.

Last July, Julian Smith came under heavy criticism when he asked multiple Tory MPs to break their pairing arrangements to take part in a knife-edge Brexit vote, “a serious breach of precedent”, says the Daily Telegraph.

Only one MP, the party chairman Brandon Lewis, followed Smith’s instructions. Lewis had been paired with the Liberal Democrat MP Jo Swinson, who had recently given birth, and “the anger in the Commons was significant”, says the Telegraph.

What is a three-line whip?

It might sound technical, but a three-line whip means the instruction on how to vote is literally underlined three times to emphasise its importance, as you would a particularly crucial item on a shopping list.

While MPs are not compelled to vote with the party for a single or two-line whip, a three-line whip means they are expected to attend the vote and toe the party line.

Permission to miss a three-line whip vote is granted only in serious circumstances and unapproved absence or a failure to vote with the party is seen as a grave offence.

What happens if an MP defies a three-line whip?

It is up to the party leader to decide whether to punish an MP, who can be dismissed from a minister or shadow minister position or even be expelled from the party.