David Cameron has insisted his EU membership renegotiation will deliver a "good deal for Britain" after predecessor Sir John Major warned against flirting with leaving the union.
The Prime Minister mounted a staunch defence of his record on getting concessions from European counterparts ahead of a crucial summit in Brussels on Thursday.
The comments at PMQs came as a poll suggested the outcome of the negotiation could have a major impact on the looming in-out referendum, which is due to be held by the end of 2017.
The ComRes research for the Open Europe think-tank found a comfortable majority - 56% to 35% - in favour of staying in the EU under existing rules, and that hardened to 65% against 26% if Mr Cameron secured all his demands.
However, if there was no agreement on protection for non-eurozone countries the "Leave" camp would move narrowly ahead, by 46% to 45%.
Although an agreement will not be reached at this week's talks, Mr Cameron is hoping to get a final deal by February.
He has refused to rule out supporting Brexit if he fails to achieve his aims.
Former PM Sir John, whose own leadership was undermined by bitter internal rows over Europe, said: "I can't put myself inside David Cameron's mind, I can tell you what my view is: my view is that this renegotiation is important but that it shouldn't decide whether or not we remain inside the European Union because of the importance of the issue."
He told BBC Radio 4's Today: "If there is anyone in the UK who ought to be anti-European and thoroughly frustrated with them then perhaps it ought to be me. I am not a starry-eyed European, I did after all say no to the euro currency in the early 1990s, I said no to the single market and in 1996 I said no to joining Schengen when it began.
"So I am sceptical of a great deal of European Union policy. But flirting with leaving, at a moment when the whole world is coming together, seems to me to be very dangerous and against our long-term interests."
He added that for the UK to "head into splendid isolation" would not be "in our interests now or, perhaps more important, in the interests of our children and grandchildren and future generations".
Asked about Sir John's warning by the SNP's Westminster leader Angus Robertson, Mr Cameron said: "What I will be doing is getting the best deal for Britain.
"This government was the first to cut the EU budget, it was the first to veto a treaty, the first to bring back substantial powers to Britain.
"We have got a great record on Europe and we will bring back a good deal for the British people."