"There will be a clear government position but it will be open to individual ministers to take a different personal position while remaining part of the government," he told the House of Commons.

The move, likely to be seen as a concession to eurosceptics, means that Cameron will not have to sack ministers who want to campaign to leave the EU for breaching the convention of collective cabinet responsibility.

Britain must by law hold the referendum by the end of 2017 but analysts say the vote could be held as early as mid-2016 if Britain's renegotiation of its relationship with the EU is completed in time.

Officials are hopeful that the terms of the renegotiation can be agreed at a summit in Brussels next month.

Cameron himself says he will campaign to stay in the EU as long as he can secure reforms to Britain's relationship with the bloc.

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