What's happening today?published at 09:12 BST
From 09:30 BST, what is known as a second reading debate will take place - which gives MPs the opportunity to discuss the main principles of the bill.
The debate cannot run beyond 14:30 BST, and a vote is expected before that cut-off time. Both sides expect the result to be close. "It's on a knife edge," one of those closely involved told the BBC.
If the bill is backed by MPs on Friday then it will proceed to the next stage known as committee, which will see possible amendments debated by MPs.
Opponents believe support for the bill has weakened since MPs last considered it. But they are not confident they have the numbers to win the vote.
Supporters and those opposed believe the outcome will come down to who turns up on Friday, a day when many MPs are typically in their constituencies rather than in Westminster.

