David Cameron has announced that Britain will vote on Thursday 23 June on whether to remain in the EU.
The prime minister said in his statement in Downing Street that he recommends remaining in a reformed EU. Theresa May, the Home Secretary, backed him in this but other ministers are likely to campaign to leave.
The London Mayor, Boris Johnson, has not yet said where he stands on the situation but leave campaigners are hoping he will join them.
Justice Secretary Michael Gove has opted to sign up to the leave campaign.