Former schools secretary Ed Balls has the best voting record on gay rights issues among the contenders for the Labour leadership.
Analysis of his voting record shows the strongest support for gay equality measures.
Mr Balls has not actually declared his candidacy yet but is widely expected to stand to replace Gordon Brown.
So far, the only Labour MPs to announce their challenges are David Miliband and his younger brother Ed Miliband.
Former health secretary Andy Burnham is also expected to announce his candidacy, while figures such as Harriet Harman and Jack Straw have ruled themselves out.
Jon Cruddas announced today that he would not stand for the leadership.
According to Stonewall research of voting on issues such as lesbian fertility rights, civil partnerships, Mr Balls has the best voting record with a score of 93 per cent.
Both Milibands are on 86 per cent while Mr Burnham comes fourth with 71 per cent.
Mr Burnham twice voted in favour of a defeated bill which said that those accessing IVF treatment must provide a father figure – effectively discriminating against lesbian fertility rights. He was also absent for three votes on gay couples adopting children.
Mr Balls and Ed Miliband were both elected in 2005, meaning they missed votes on gay adoption and civil partnerships.
Stonewall chief executive Ben Summerskill said: “We hope everyone involved in choosing the next Labour leader will carefully examine the track records of each candidate on equality.
“How people have voted in the past is an important indicator of whether they can be trusted to take equality for everyone in Britain seriously in the future.”
Today, acting Labour leader Ms Harman said the new Labour leader will be declared on September 25th at the beginning of the party’s annual conference.
Ruling out a quick election, she said the four-month contest would be “Labour’s opportunity to re-engage with the British people”.