Shadow Home Secretary David Davis quit as an MP to fight a by-election on the issue of 42-day detention.
Who is David Davis?
One of the most experienced and long-serving Conservative frontbench politicians in the Commons, who has twice challenged unsuccessfully for the party leadership. Often portrayed as a "hard man" and a bruiser he was always seen being on the right of the party and a hardliner on law and order. But he has also become a passionate defender of civil liberties since taking over as shadow home secretary in 2003.
Why has he stood down as an MP?
He says he wants to force a by-election in his Haltemprice and Howden constituency, to campaign against "the slow strangulation of fundamental British freedoms by this government". In particular, he wants to take a stand against government plans to extend the maximum time terror suspects can be held by the police to 42 days.
Didn't MPs vote on 42 day detention on Wednesday?
They did. The government won by the slenderest of margins - but Mr Davis said they had "bought" the nine votes they needed to get it through the Commons. He believes MPs have failed to protect traditional liberties and the only way to reverse this "monstrosity" is to return to Parliament with a mandate to overturn it.
Is this resignation unusual?
Yes, very. There are very few precedents in British politics. In 1986 all 15 Ulster Unionist MPs resigned and provoked by-elections in protest over the Anglo-Irish Agreement. In 1910 Independent MP George Lansbury quit to fight a by-election in favour of votes for women. He lost but later became Labour leader. John Major quit as Tory leader (but not as an MP) in 1995 in order to hold a "back me or sack me" leadership election. But he was trying to bolster his weak position - Mr Davis was one of the most effective front bench performers in the Commons and was seen by many as being on course to be the next home secretary.
Can MPs just quit like that?
Yes. But instead of handing in their notice, they are appointed one of two jobs which excludes them from membership of the Commons. These are the Crown Steward and Bailiff of the Manor of Northstead and the Chiltern Hundreds. Mr Davis is expected to take the Chiltern Hundreds on Friday or Monday.
When will the by-election be held?
It will be up to Tory party managers - but local party contacts in Haltemprice and Howden say it will probably be on 3 July or 10 July.
How much will it cost?
According to the Ministry of Justice, the average cost to the taxpayer of a by-election is £70,000 to £80,000, although it varies depending on the size of the turnout and the constituency.
Will Mr Davis win?
Almost certainly. The Lib Dems - who also oppose 42 day detention - say they will not be fielding a candidate against him. Some Labour MPs are urging their party not to contest the seat in order to scupper what they have described as a "stunt".
What does this mean for Tory leader David Cameron?
It is hard to say. With the Conservatives riding high in the opinion polls, it will be seen by some as a major own goal. Labour claim it shows they are not fit to govern. Prime Minster Gordon Brown believes the public are behind them on 42 day detention but some voters may see Mr Davis' resignation as a principled stand.
Doesn't Mr Cameron support Mr Davis on 42 days?
Yes. He has also attacked the government's "appalling" decision to push ahead with it. Mr Davis' successor as shadow home secretary, Dominic Grieve, has said he would repeal the legislation if the party wins the next general election.
So has Mr Davis fallen out with Mr Cameron over some other issue?
There are no reports of any disagreements. Mr Davis was runner up to Mr Cameron in the 2005 leadership contest, but they appeared to have buried any policy differences they had and were thought to be working well together.
So will Mr Davis be an official Conservative candidate?
Yes. David Cameron says he will campaign for Mr Davis at the by-election, but he will not receive any financial support from Tory HQ.