FILE PHOTO: more than a quarter of the parents who said their relationships were in trouble but remained together described themselves as "extremely happy" ten years later.

London - Two out of three couples whose relationships are on the rocks ultimately find happiness if they stick it out, a report claimed.

It found that 68 percent of parents who said they were happy just after the birth of their first child went on to say they were content together a decade later.

The research undermines the widely-held belief that many couples remain trapped in unhappy relationships for years for the sake of their children.

The report was based on parents who said they were unhappy when asked about their relationship shortly after the birth of their first child around the turn of the millennium, and who were then questioned again in 2011. It found that one in 20 new parents say they are not happy with their relationship.

Of those, seven out of ten stay together, with only one in ten of the couples who remain partners still reporting unhappiness a decade later.

And more than a quarter of the parents who said their relationships were in trouble but remained together described themselves as "extremely happy" ten years later.

Surprisingly, the results also showed that the most vulnerable relationships were not of those couples who said they were miserable. Instead, those who said they were not sure whether they were happy or not were more likely to break up.

The research, carried out by Harry Benson of the Marriage Foundation and Steve McKay of the University of Lincoln, found that cohabiting couples who said they were on the brink of break-up were more than twice as likely as married couples actually to part.

But, their report said, there were major rewards for parents who carried on through the rough patches.

They estimated that only one in 400 couples are unhappy after the birth of their first child and remain unhappy after staying together for ten years.

The findings, based on data from 10 000 parents who took part in the state-backed Millennium Cohort Study, undermines family lawyers who urge unhappy couples to break up on the grounds that conflict at home is worse for children than divorce.

Mr Benson said: "Contrary to popular belief, staying in an unhappy marriage could be the best thing you ever do.

"Most marriages have their unhappy moments, but apart from the fortunately extremely rare cases where the relationship involves abuse, most couples can work through the difficulties to be happy later on."

Marriage Foundation head Sir Paul Coleridge said the research was "myth-busting", adding: "Because a couple is having a tough time adjusting to the demands of children does not mean they will not come through it and end up with a really high quality relationship."