Britain will vote to reject a new deal with Brussels unless the European Union agrees to "substantial" permanent reforms, the Foreign Secretary has warned.
Philip Hammond said the British public would deliver "a raspberry" to the Government if David Cameron's efforts to renegotiate the relationship with the EU did not secure "robust" changes.
Both Mr Hammond and Business Secretary Sajid Javid stressed that the Prime Minister had ruled nothing out - indicating he could be prepared to recommend a British exit from the EU if he failed to secure key reforms.
Work and Pensions Secretary Iain Duncan Smith said the twin crises of mass migration and Greek debt have hit the EU like an "an out-of-control bulldozer" and made leaders on the continent more receptive to Mr Cameron's drive for reform.
As Tories began to gather in Manchester ahead of the first autumn conference under a Conservative majority government since 1996, the issue of Europe looks set to dominate the agenda.
Mr Cameron has promised an in/out referendum by the end of 2017 and Mr Hammond said the outcome rested on Brussels' willingness to meet the Prime Minister's demands.
Speaking to The Daily Telegraph Mr Hammond said: "If we can't get the commitments we need from our European partners on things like Britain being outside the commitment to ever-closer union ... If we can't get these things then, as the Prime Minister has said, we rule nothing out.
"That's why the package will have to be a robust, substantial and irreversible package of change with proper binding legal force. Because if we try to put to the British people a package which is anything less, we will get a raspberry from them."
He suggested the Cabinet would have a say in the final recommendation of whether to recommend staying in or leaving the EU.
"The job for the Cabinet will be to look at the deal that we're able to negotiate and decide if - on balance - it is in Britain's best interest to stay in a European Union, reformed, as per the deal we've negotiated or not."
Mr Javid, who like Mr Hammond and Mr Duncan Smith is viewed as a Eurosceptic, told the Daily Mail: "The right approach now is to put all our efforts into getting the changes that we want to see and increasing the chance of getting those.
"We will then look at what we have achieved and that's the time to make up our mind up.
"But remember, the PM has also said nothing is off the table."
Following the crises that have rocked Europe, Mr Duncan Smith told the Guardian: "We are getting a better hearing because people are waking up to these things. It is suddenly becoming clear that actually you cannot paper over the cracks and say 'it's alright, it's only the British'.
"We still have the crisis over the euro and Greece, and then the rows over Schengen border controls are like nothing I have ever seen. It is massive."
Mr Duncan Smith, who claimed the Germans were "petrified" by the prospect of Brexit, added: "Nothing is the same after this thing. The European Union has just been hit by an out-of-control bulldozer that has just gone straight through the middle of them."
Mr Hammond, who was this week in New York for the United Nations general assembly, revealed that Britain's allies had been "bamboozled" by the election of Jeremy Corbyn as Labour leader.
Mr Corbyn's first conference as leader ended in a major split over his commitment to unilateral nuclear disarmament and his revelation that he would never press the button to launch a nuclear strike if he was prime minister.
Mr Hammond said the Labour leader's stance on foreign policy and defence "embarrasses Britain and it weakens Britain".
He added: "To the extent that people overseas are becoming aware of these very specific and unwise positions that he has espoused, they are aghast.
"I can't tell you the reaction in the US to hearing that the leader of the official Opposition in Britain said that the death of Osama bin Laden was a tragedy."