Frontbenchers who had already agreed to repay money include George Osborne, Michael Gove and Alan Duncan.

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Mr Carswell has written to the prime minister, challenging him and frontbenchers of all sides to support his Ten Minute Rule Bill.

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Members are appointed for the full parliamentary term, though changes are fairly common - for instance, when members become ministers or frontbenchers.

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There are certainly individuals like frontbenchers George Osborne and David Cameron who are seen as amongst the brighter hopes of the future.

BBC: Analysis

Ministers are also described as frontbenchers, since they occupy the front row of seats on their side of the House when they attend debates.

BBC: Democracy Live site links

Yet Labour has spoilt a good case by setting up a blind trust to finance Tony Blair's private office (as well as similar trusts for other frontbenchers).

ECONOMIST: Party funding

We have had some nincompoop frontbenchers and some expense-fiddling backbenchers, and even some who managed to qualify under both categories, but our political system is basically sound.

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MPs from the main opposition parties who take on shadow ministerial positions are allowed to speak in the Commons from the front row of seats, hence the distinction drawn between "frontbenchers" and "backbenchers".

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Labour peer Lord Barnett, former Chief Secretary to the Treasury, said there was "not a cat in hell's chance" of the Financial Services Bill becoming law and urged frontbenchers to work together on a consensus.

BBC: Barnett: Financial Services Bill will not become law