John Bercow, the Shadow Work and Pensions Minister, has resigned over the Conservative Party's opposition to allow gay or unmarried couples to adopt children, in advance of a crucial vote on the issue today. Philip Webster, left, Political Editor, reports on the resignation.

Who is Mr Bercow?

He used to be associated with rightwing groups in the party - he was a member of the Federation of Conservative Students and he had an association with the Monday Club.

But in recent years he has undergone a transformation and is now seen as being on the Left, especially when it comes to social issues such as Section 28 and allowing unmarried and gay couples to adopt children.

He also is a prodigious tabler of parliamentary questions. Earlier this year it was calculated that he spent £552,672 of taxpayer's money by tabling questions – 2,619 in all - to government ministers on anything from youth unemployment to crèche facilities for Government staff.

Why has he changed his tune?

It's not so much that he has changed his tune on social issues - he has always been on the libertarian right of the party. But, after the crushing defeat of 1997, Mr Bercow, along with prominent modernisers such as Michael Portillo, were adamant that the Conservative Party had to take a long hard look at itself if it was ever to have a chance of being re-elected in the medium to longterm.

He has been on the edge of trouble for some time. He was demoted in the last shadow cabinet reshuffle, from Shadow Chief Secretary to the Treasury to Shadow Work and Pensions Minister, probably as a punishment for his alliance with the Portillistas.

Was his resignation expected?

No, it was quite a surprise to us all. Mr Bercow, along with other members of the Shadow Cabinet, had been given more than ten days' notice by Mr Duncan Smith that they would be required to vote against government proposals to allow gay and unmarried couples to adopt.

If they felt that they could not do so, they could make themselves absent from the House on the day. Mr Bercow did this on the first vote, but felt that he couldn't do so again, so he resigned as a matter of principle.

What damage will this inflict on Iain Duncan Smith?

This is exactly what Mr Duncan Smith did not need. It shows a lack of political judgment on his behalf and that the Tories' claim to be modernising is somewhat hollow. They signalled that they were willing to change at their conference last month, but then Mr Duncan Smith instructs MPs to vote against gays and unmarried couples adopting children. The Labour Party will be able to say that the Tories are forever stuck in the past and that it is still the "nasty party".

Will this re-ignite a challenge to Mr Duncan Smith's leadership?

No. There has been quite enough of that recently. Michael Portillo, the most obvious challenger, has made it very clear that he is not interested in standing as leader. However, episodes such as this might well harm Mr Duncan Smith further down the line if any challenge is forthcoming, because it he displayed a lack of political judgment.

Mr Duncan Smith could have made it easier for his MPs by allowing them a free vote tonight, like Labour has done. But, by making forcing his MPs to vote against the plans, he invited trouble.

What it the latest on the "gay adoption" Bill?

The Adoption and Children Bill is now in its final stages. It is coming back to the House of Commons today having been amended in the House of Lords. The Lords rejected the proposal to allow gay or unmarried couples to adopt, and the Commons is now expected to insist that proposals such as the one at the centre of Mr Bercow's resignation are included in the legislation.

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