Cabinet minister John Whittingdale has indicated he stands ready to campaign for Britain to quit the European Union.
The Culture Secretary pointed to his track record of being "highly critical" of the way the 28-strong bloc is run as he refused to rule out backing a Brexit.
It comes as David Cameron faces further pressure to produce measures aimed at boosting the powers of Westminster over Brussels amid continued criticism from Europsceptic Tories about the reform proposals unveiled on Tuesday.
Mr Whittingdale told The House magazine: "My position is that the Prime Minister is out there trying to negotiate the best deal he can for the country. I have a track record where I've been highly critical of the way the EU works and I have opposed measures for closer integration and it certainly needs reform.
"I hope the Prime Minister will get that agreement and then I'll look at it when he comes back with it."
Asked if he would rule out backing Brexit, he replied: "I wouldn't."
Critics claimed the proposed emergency brake on benefits is "highly unlikely" to stop EU migrants heading to Britain after records showed "quite low" numbers claimed tax credits.
Some 84,000 EU migrant households who arrived in the UK or been issued a National Insurance number during the previous four years received the benefit in 2013/14, according to HM Revenue and Customs.
Figures released under Freedom of Information laws showed that 50,000 were claims from couples and 34,000 were single claims.
Proposals for reforming Britain's relationship with Brussels include an emergency brake to restrict benefits for migrant workers, with benefits phased in over the four-year period.
Jonathan Portes, principal research fellow at the National Institute of Economic and Social Research, said: "This new data shows that the Prime Minister's claim that 40% of recently arrived European migrants were dependent on benefits was at best selective and misleading.
"Given that well over a million European migrants registered for National Insurance numbers over the period in question (and far more since) this suggests that claim rates among newly arrived migrants are, as researchers have always argued, quite low.
"Once again this suggests that the PM's focus on this issue is misguided - the 'emergency brake' will have only a modest impact on benefit receipt, and is highly unlikely to have a significant impact on migration flows.
"This latest data still leaves some important questions unanswered. In particular, the Government is still refusing to tell us - for obvious political reasons - how many recent European migrants are recorded as being active in the UK labour market."
The emergency brake proposals and plans for a "red card" allowing parliaments to block laws from Brussels have been further ridiculed by Eurosceptic Tories, with claims they are a "desperate attempt to put lipstick on a pig".
Mr Cameron has been meeting key players from the EU in London, holding talks with European Council president Donald Tusk, Slovak PM Robert Fico, Greek PM Alexis Tsipras, Swedish PM Stefan Lofven and Belgian PM Charles Michel, in the margins of the Syria donors' conference.
Number 10 said leaders had agreed to "continue working together" to find "solutions" during the intense round of talks.
Downing Street said the Prime Minister will meet with the heads of the political groups in the European Parliament before the crunch meeting of EU leaders to discuss the reform proposals later this month.
After discussions in Downing Street, European Parliament president Martin Schulz said a "compromise" could be found.
Speaking outside Number 10, he said: "I think there is enough good will for a compromise and still a lot of things to discuss.
"But in the end I think we will find a compromise to which UK citizens can vote to stay in the European Union."