9.01am GMT

One of the abiding themes of British politics over the last 200 years or more has been the battle between the Tory party and the modern world. Today we are going to witness another instalment. The modern world is going to win - but not without a fight.

MPs are debating the marriage (same sex couples) bill, the government legislation that will allow gay marriage (or equal marriage, as some campaigners prefer to call it, although I think gay marriage is clearer). The bill will get approved by a large majority because Labour and Lib Dem MPs are overwhelmingly in favour, but David Cameron’s party is virtually split down the middle. There are two key questions to be resolved.

1. How large will the “rebellion” be? I’ve put rebellion in inverted commas because MPs have been given a free vote, and so technically MPs who vote against are not actually rebelling. But they will be dissenting from Cameroon modernisation, and so I think the “rebels” label is a fair one. As Nicholas Watt explains in the Guardian today, after the vote on the second reading at 7pm, there will be a second vote on the bill’s timetable motion. This second vote is whipped, and so we could see a real rebellion then. It will also be worth counting the Labour “rebels”. Ed Milband and his shadow cabinet are all voting in favour of gay marriage, but party sources say around 25 Labour MPs will vote against.

2. How much damage will this do to Tory unity? Sometimes a party can split on an issue and then swiftly move on. (Apart from Jesse Norman, can you remember who the Tories were who rebelled over Lords reform? If it had not been for the link with the boundary changes, that vote would have already been relegated to ancient history in the folk memory of the party.) And then sometimes a party can split and the repercussions can last for a decade or more, with loyal voters walking away, activists leaving and internal party selections being swayed by the outcome. Look at the Iraq war vote and its effect on Labour. Today’s vote could be just as damaging to the Conservative party. It’s hard for anyone with a youngish, liberal, urban background to understand what all the fuss is about - as Polly Toynbee says in a great column today, “we can understand why a minority of people are profoundly upset by abortion, but this arcane marriage dispute is beyond the ordinary comprehension of anyone not guided by the Bible” - but the opponents of gay marriage feel extraordinarily strongly about this, as was revealed when Maria Miller, the culture secretary, first unveiled the plans in the Commons. It's worth trying to understand why, because the consequences of this could be considerable.

I will be covering the debate in detial, and focusing on the issue all day. But there are some other items on the agenda. Here’s the diary.

9.30am: The cabinet meets.

9.30am: Christopher Graham, the information commissioner, gives evidence to the Commons justice committee.

10am: Vicky Pryce, Chris Huhne’s ex-wife, goes on trial over claims she took his speeding points.

10.30am: Hacked Off and Press Council of Ireland give evidence on press regulation to the Commons culture committee.

12pm: Joe Biden, the US vice president, arrives at Downing Street for talks with David Cameron.

12.30pm: MPs begin debating the second reading of the marriage (same sex couples) bill. The vote will come at 7pm.

I’ll post a lunchtime summary at around 12pm, another in the afternoon and another after the debate is over.

If you want to follow me on Twitter, I'm on @AndrewSparrow.