UPDATE: The entire 4 hour debate is here
Update: the House of Commons passed the legislation 400-175.
BBC is live streaming the marriage equality debate today.:
The bill will legalise gay marriage and enables same-sex couples to get married in both civil and religious ceremonies, where a religious institution had formally consented, in England and Wales.
The bill will also allow couples who had previously entered into civil partnerships to convert their relationship into a marriage.
Conservative and Labour MPs have been given a free vote on the bill but the debate around same-sex marriage has opened up significant differences within the Conservative party - up to 150 Tory MPs are expected to defy the prime minister and vote against the bill.
But a group of 50 senior Conservative activists have warned that opposing gay marriage could "risk alienating the voters we will need in 2015".
Watch it here