As the British Cabinet meets to discuss the deal David Cameron secured in Brussels - it is thought ministers will begin announcing if they will campaign to stay in or leave the European Union.
Prime Minister David Cameron has already accepted his close ally, Justice Minister Michael Gove, will vote for Britain to leave the EU.
Mr Cameron said: "Michael is one of my oldest and closest friends.
"He has wanted to get Britain to pull out of the European Union for about 30 years, so of course I'm disappointed that we're not going to be on the same side as we have this vital argument about our country's future.
"I'm disappointed but I'm not surprised."
It is thought that Mr Gove will not take a high-profile in the Brexit campaign out of his loyalty to the Prime Minister, Sky News reports.
Leader of the Commons, Chris Grayling and Work and Pensions Secretary Iain Duncan Smith are both expected to be among the first to announce their support for the Out campaign.
Theresa Villiers, the Northern Ireland Secretary, and John Whittingdale, the Culture Secretary, are also thought to join the Out campaign.
Frontbenchers Chancellor George Osborne, Home Secretary Theresa May are set to support the Prime Minister and the In campaign.
London mayor Boris Johnson has spoken out about his irresolution regarding the referendum.
