Why is an MP required to comply with a Three Line Whip?
We already have a question about why MPs are required to comply with party whips in general. But I would like to know why is an MP required to comply with a Three Line Whip in particular.
I've just seen a post from MP for Tottenham and former Minister for Higher Education David Lammy on his Facebook account saying that he received a reprimand for not complying with Chief Whip on the division(s) of the European (Notification of Withdrawal) bill.
"You are required to comply with the Whip in the future"
Why would an MP be required to follow what a Three Line Whip asks of them? or are there special circumstances for doing so?