The initial round of appointments to select committees happens in the first few weeks or months after a general election, after the chairs have been separately elected. In most cases, once appointed you can stay on a committee for the rest of the Parliament if you choose to. The most significant exception is the Backbench Business Committee, whose members must be re-appointed each session.
But the membership of committees changes continually for many different reasons and vacancies are filled by some form of ‘by-election’. In addition, sometimes a new committee will be set up and the places on it will have to be filled.
If you want to leave a committee, you should write to the chair and the clerk to let them know and you should also notify your party’s whips. Formally, you remain a member of the committee until the House has agreed a motion to discharge you, which doesn’t usually happen until someone has been found to replace you.
With a very few exceptions, ministers are not members of select committees. In addition, it’s the custom that Opposition frontbench spokespeople don’t become members of select committees that scrutinise the work of the Government, although they might serve on committees that look at the way the House is run. Slightly different conventions apply to the smaller Opposition parties. There is also a strong convention that parliamentary private secretaries don’t sit on committees scrutinising departments or areas of policy for which their ministers are responsible.
If you’re appointed to a ministerial or other post that conflicts with your role on the committee, you will normally be expected to stop any committee work immediately. You should ask your whips to arrange for your formal discharge and replacement as soon as possible.