How does MP pairing work in Parliament votes and what is a three-line whip?

A ROW erupted in Westminster after Tory whips were accused of breaking a deal to "pair" MPs for a crucial Brexit vote.

Last summer Tories were accused of cheating when an MP voted after agreeing not to while a Lib Dem was on maternity leave. Here's our guide to the Parliamentary custom.

What is MP pairing?

Sometimes MPs are not able to make it to the Commons for votes - for instance if they are ill, or away on ministerial or committee business.

This affects all parties, not just the one in government.

If a vote looks like it will be close, there could be huge pressure on MPs to race back to London or rise from their sick beds.

To avoid this, party whips - who are in charge of discipline and making sure MPs vote - arrange deals with other parties.

Two opposing MPs who would otherwise cancel out each other's votes are "paired", meaning they both agree not to turn up for a particular division.

Breaking a pairing arrangement is considered a serious breach of honour.

And pairings with pregnant women and new mothers are sacrosanct.

Pairing is an informal arrangement which is not officially recognised by the House of Commons, but must be registered with the whips.

Generally, pairing is not allowed in divisions of the greatest importance when there is a three-line whip.

There have been calls to replace it with a system of proxy voting for MPs who cannot be physically present.

Last summer this system was put under scrutiny when Lib Dem Jo Swinson was denied a vote when the Government broke a pairing arrangement when she was heavily pregnant.

What is a three-line whip?

A three-line whip is an instruction given to MPs by the leader of their party to vote on an issue in a certain way.

It is the most serious, above a single line which is issued as guidance, and double line which requires MPs to attend.

Any MP who doesn't follow the three-line whip risks having their position put in question.

Whip's within each political party are required to ensure that MPs follow the voting order.

On February 27, 20 Tories defied a three-line whip issued by Theresa May, as they voted against the Cooper amendment.

And an extra 88 Tories did not even vote.

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