DOMINIC GRIEVE, 62
Mr Grieve is a former attorney-general who has already waged successful battles to secure a greater role for Parliament in the Brexit process. He is also the reason Mrs Theresa May has come back to Parliament with a Plan B just three days after her deal was thrown out.
Mr Grieve wants a second referendum and is using all his experience as a lawyer and politician to make it happen.
OLIVER LETWIN, 62
Mr Letwin is a former minister who has already brokered deals between the government and pro-EU Conservative rebels in Parliament.
Although traditionally a May loyalist, he voted against the government this month to try to prevent a no-deal Brexit.
He has spoken in favour of a Norway-style agreement that would keep Britain inside the single market.
HILARY BENN, 65
Mr Benn is the pro-EU chair of Parliament's Brexit Committee. He served in the Cabinets of former prime ministers Tony Blair and Gordon Brown and is not an ally of Labour leader Jeremy Corbyn.
He is fighting to prevent the chaos of a no-deal departure and maintain closer ties with the EU than Mrs May's plan allows.
He has proposed what is potentially one of the most important motions of this week's debate - a wrecking amendment that calls for Mrs May's withdrawal agreement to be thrown out and for the government to reject a no-deal exit.
YVETTE COOPER, 49
Ms Cooper is another former Labour Cabinet minister who is scrutinising the Brexit details. She is also not an ally of Mr Corbyn, but an outspoken critic of the government's policy, particularly when it comes to the impact leaving the EU could have on national security.
She inflicted a bruising defeat on the government on Jan 8 with an amendment that would curtail the government's tax powers in the event of no deal.
JOHN BERCOW, 55
Speaker of The House of Commons
Mr Bercow is perhaps the most controversial Speaker in Britain in recent memory.
His relations with Mrs May's office have long been strained amid accusations of bias, but they hit a nadir last week when he made an unprecedented decision to allow an amendment proposed by Mr Grieve - to Mrs May's detriment.
Those who are trying to soften Brexit see Mr Bercow as an ally.
Mr Bercow, who voted remain in the 2016 referendum, has a reputation for giving rank-and-file Members of Parliament time during key debates.
It is up to the Speaker to select amendments, giving him power to shape the vote today.
KEIR STARMER, 56
Mr Starmer, once a high-flying lawyer, is the Brexit spokesman for the opposition Labour Party. He represents one of the most remain-voting constituencies in Britain and is more pro-EU than the Labour leader.
He is credited with slowly shifting Mr Corbyn's Brexit policy to a softer position, and wants to maintain close ties with the bloc.
He was instrumental in forcing the government to publish Brexit documents it wanted to keep private, and has worked with Tory colleagues to try to prevent a no-deal Brexit.
BLOOMBERG, AGENCE FRANCE-PRESSE