Gay marriage is against the law in many countries
Gordon Brown is pushing for British civil partnerships to be recognised across the EU - including Eastern Europe, he has told Attitude magazine.
The prime minister said Britain was negotiating deals with France and Spain but wanted to "go further than that".
He told the gay lifestyle magazine it was important to show that "respect for gay people was due" in "Eastern Europe as well as Western Europe".
But he predicted it would take "many years" for this to happen.
Campaigners have been pushing for people who have undergone a civil partnership to be granted the same legal status and rights they enjoy in the UK in every other EU country.
A lack of legal recognition can affect issues such as immigration and pension rights.
Mr Brown was also quizzed by Attitude about his own voting record on gay rights - he missed several votes on the issue during the first eight years of the Labour government - and what he was doing to tackle homophobic bullying in schools and the police, which campaign group Stonewall claims is on the increase.
He said he had probably been busy with Treasury business when the votes were taking place but added: "I've always given my strong support."
On tackling bullying, he said: "I want to do much better. In every area where you've got prejudice or you've got discrimination, you have to change the laws, but you also have to change the culture."
But he refused a call to cut British aid to Uganda, where there are proposals to reintroduce the death penalty for homosexuality, saying he regularly raised the issue with the Ugandan government but "the point about aid is that it saves lives".
In an interview with journalist Johann Hari in this month's edition of Attitude, Mr Brown hails civil partnerships as a key Labour achievement, which "showed our country is far more tolerant than people thought".
He told the magazine: "I'm fighting to get all the countries in Europe to recognise civil partnerships carried out in Britain.
"We want countries where that hasn't been the case - especially in Eastern Europe - to recognise them. We're negotiating agreements with France and then with Spain," said Mr Brown.
"But I think we can actually go further than that. And if we could show Eastern Europe as well as Western Europe, that this respect for gay people is due, that would be really important.
"Of course it will be tough, and will take many years, but that has never ever been a good reason not to fight."
All three main parties at Westminster have been competing for the gay vote in recent years.
The Private Members' Bill that paved the way for civil partnerships was introduced by a Liberal Democrat peer, Lord Lester.
Earlier this year, David Cameron won plaudits from campaigners after he apologised for Section 28 - the Conservative inspired legislation banning the "promotion" of homosexuality in schools.
But the Conservative leader has also come under fire for forming alliances with allegedly homophobic Polish politicians in the European Parliament - something firmly denied by Mr Cameron but a theme returned to by Mr Brown in his Attitude interview.
Gay marriage and same sex adoption are banned across most of Eastern Europe and in some countries, such as Romania, homosexuality has only been decriminalised in the past decade as a condition of joining the EU.