A continuing decline in marriage has been recorded in the UK’s 2021 census figures, prompting a clash about whether to bolster the institution of marriage or enhance the status of cohabitation.

The biggest fall came among people aged 25 to 35, with 1.2 million more people unmarried in that age bracket than there were in 2011.

The slump has sparked fresh calls to protect rights for those not in legally registered partnerships, a move which would further blur the distinction between married couples and cohabitees.

The Marriage Foundation said the social pressure to marry in the UK had “pretty much disappeared”, despite “the psychology of marriage remaining deeply compelling”.

Harry Benson, the charity’s research director, argued that the process of making an active decision to be together tended to increase commitment and remove ambiguity, which was “one of the biggest relationship killers”. Getting married in front of your peers also creates “accountability”, he said.

But he said marriage was falling because it had come to be seen as optional.

He said government messaging often projected cohabitation and marriage as equal, while an “irresponsible marriage industry that claims the average cost of a wedding is £30,000” was deterring people. He also highlighted how the welfare system limited payments to married people.

“The decline of marriage is much less obvious among high income groups,” Benson said. “The miracle is that anyone in lower income groups marries at all with this huge couples penalty.”