Philip Hammond said he "can't envisage" breaking ranks to campaign against European Union membership if David Cameron secures a renegotiation deal.
The Foreign Secretary - who has previously said he would back "Brexit" if the relationship is not changed - said he would not take advantage of the Prime Minister's decision to free Cabinet ministers from collective responsibility.
"Our first challenge is to get the right deal, and we are making good progress on that," he told BBC Radio 4's Today after Mr Cameron expressed confidence of securing backing for welfare curbs on the latest leg of an intense diplomatic tour.
"It is a painstaking task because we've got 27 partners to bring along with us.
"Then the Government will make a decision about whether it can make a recommendation to the British people on the back of that deal to support Britain's continued membership of the European Union.
"I can't envisage us negotiating a deal which the Prime Minister thinks is good enough to recommend to the British people and which I feel I want to campaign against. I can't envisage that circumstance."
The PM insisted on Thursday that he still hopes to complete his EU membership renegotiation next month after his Hungarian counterpart said he was "sure" British concerns about benefits abuses could be accommodated.
At a joint press conference after talks in Budapest, Viktor Orban sharply denied that Hungarians were "parasites" on the UK taxpayer.
But he said he recognised anxiety over "abuse" of Britain's welfare system, and expressed confidence that the V4 - Hungary, the Czech Republic, Poland and Slovakia - would agree to a solution.
The comments will have encouraged the Prime Minister at the end of another gruelling diplomatic offensive which saw him meet Angela Merkel in Bavaria on Wednesday evening before heading for Budapest.
Mr Cameron said his proposal of a four-year ban on migrants claiming in-work benefits - viewed as the most difficult part of the reform package - was still "on the table" although he reiterated that he was ready to listen to alternatives.
Speaking through an interpreter, Mr Orban avoided addressing the specific proposal and said he would not accept any "discriminatory" measures.
But he added: "I think we will be able to agree ... "I am sure we are going to be able to find a solution that is going to be suitable for the Hungarian employees."
He said: "The abuses that are seen in social benefits systems have to be eliminated. I made clear that the Hungarian government does not support any abuses at all."
The Daily Telegraph reported that EU leaders were offering Mr Cameron a deal that would see the ban extended to 18 to 22-year-old British workers but allow the impact on them to be mitigated through other payments.
A Downing Street source said: "There's been lots of noise and speculation around our renegotiation in recent months - and this is just the latest example."
Mr Cameron says the Government will make a "clear recommendation" on whether the UK should stay in the 28-nation bloc or leave, following the conclusion of the renegotiation of the terms of its membership.
But he told MPs that it would be open for individual ministers to oppose this recommendation without quitting their Government posts, in a significant departure from the usual principle of collective responsibility.
The move has thrown a spotlight on senior Tories such as Home Secretary Theresa May, Work and Pensions Secretary Iain Duncan Smith, Leader of the Commons Chris Grayling and London Mayor Boris Johnson, who will face increased pressure to declare which way they will vote.