Most children in the UK will be born outside marriage within three years, new official figures suggest.

According to new data from the Office for National Statistics, nearly half of all children in England and Wales, 47.5pc, were born outside marriage in 2012.

The comparable figure was 25pc in 1988 and just 11pc in 1979. The figures led to growing calls from Conservative backbenchers to introduce a tax cut for married couples, the Telegraph reports.

Research suggests that children whose parents married tend to be happier and achieve more success later in life educationally and financially.

‘This continues the long-term rise in the percentage of births outside marriage/civil partnership, which is consistent with increases in the number of couples cohabiting rather than married or in a civil partnership,’ the ONS said.

Mr Cameron has promised gay and straight married couples will benefit from a tax break, worth around £150-a-year.

Tim Loughton, the former Children’s minister, urged the government to introduce the allowance to halt the decline in marriage.

“If people are prepared to make a public declaration to each other in front of their friends and family they are more likely to stay together. Without marriage people drift in and out of relationships very easily,’ he told the Telegraph.

“In families where parents break up children do less well at school, are more likely to suffer mental health problems and are more likely to have substance abuse problems.

“The government needs to send a very clear message that it supports marriage. That’s why married tax breaks are so important.”

His words were echoed by, Christian Guy, the director of the Centre for Social Justice, a think-tank founded by former Tory leader and current Secretary for Work and Pensions Iain Duncan Smith.

He said: “Marriage is not a right wing obsession, but a crucial social justice issue. People throughout society want to marry but cultural and financial barriers faced by those in the poorest communities thwart their aspirations.

“Evidence shows quite clearly that children growing up with married parents tend to have better life chances. The Government must deliver on on its family friendly pledge.”

Last year, 346,595 babies were born outside marriage.

Over the last three-quarters of a century the number of babies born in England and Wales has fluctuated.

In 2012 there were 729,674 live births, up around 6,000 on the year before, continuing a rising trend seen since 2001 when there were only 594,634.

Mothers now have an average of two children, the highest fertility rate since the 1970s.