David Cameron is facing a split in his party as he attempts to renegotiate Britain's relationship with the European Union.
With a slim majority in the House of Commons, and a referendum battle to face after his negotiations, he may face future rebellion in his own cabinet over the EU.
Some 50 Tory MPs have joined the Conservatives for Britain group in Parliament, campaigning to keep pressure on reform. Its co-chair, Steve Baker MP, has said that he "wouldn't be surprised" if "one or two" cabinet ministers resigned if "we don't get a sovereign Parliament".
Cameron has said that he expects government ministers to follow the party line, so who could be the ministers to resign to campaign for a Brexit?
Iain Duncan Smith - Secretary of state for work and pensions
Smith, a renowned eurosceptic, defied his party under the Major government in order to oppose perceived pro-European policies.
Last year he said the prospects of Britain's continued membership of the EU were "very bleak" if Europe did not embrace reform. He added:
We tend historically, no matter who is in government – Tony Blair or whatever – to make the big noises in Europe because we say it as it is quite often, and don't do all of these behind the scenes deals. The truth is it [the EU] needs at the moment, as the prime minister is doing, to be roughed up a bit about this.
Michael Gove - Secretary of state for justice
In 2013, Gove said he would vote to leave the EU if a referendum was held then. He said a Brexit would lead to "certain advantages" for the UK.
The justice secretary has said his party will not be forgiven for "fudging" a referendum, adding it was a "historic opportunity" that must not end in the UK being "steamrolled into a reluctant, strangulated and acquiescent yes" by the EU.
Chris Grayling - Leader of the House of Commons
Grayling was the author of the Conservative Party's policy on the European Convention on Human Rights, which recommended leaving the European model in favour of a British Bill of Rights.
Writing on Conservative Home about the ECHR, he said:
As Conservatives, it falls to us to put Britain first, and restore common sense to our human rights framework. As a majority Conservative government, that is just what we would do.
If EU negotiations are modest, the eurosceptic Grayling could wrestle over his conscience, campaigning instead to "put Britain first".
Oliver Letwin - Cabinet minister and policy chief
Mr Letwin has previously said he wanted to avoid Britain being absorbed into "a United States of Europe", adding that he was "not 100 per cent" sure Cameron could successfully renegotiate Britain's relationship with the EU.
He has tied his support for the EU to the implementation of reform, telling students at University College London that the likelihood of this was "better than 50/50". He said:
If we can achieve that in negotiations then I shall certainly be supporting remaining within it in the referendum in 2017. We have to have that referendum. We have to put to the British people that question.
Priti Patel - Minister of state for employment
Patel may also wrestle with her conscience if Cameron fails to achieve significant reform in Europe.
Writing on Conservative Home last year, she said Europe "isn't working", adding:
The British public want our political leaders to say "No" to the unaccountable federalist European agenda which has led to the widespread failures of the European Union we face today.