Two dating websites for men and women seeking polygamous marriages have attracted 70,000 members, according to their founder, who says he believes that they are responsible for at least 100 weddings.

Secondwife.com was launched in 2014, followed by Polygamy.com this year, to match married men seeking more wives with women prepared to be in a multiple relationship.

The websites came under attack this week in Dame Louise Casey’s report on social integration. She cited them as an example of how values contrary to the British way of life were being tolerated.

“The practice of unregistered polygamy appears to be more commonplace than might be expected,” Dame Louise said. “The existence of matchmaking sites like secondwife.com and the prevalence of unregistered marriage is particularly concerning.”

Her report said that there were about 100,000 couples living in Sharia marriages, many of which have no basis in UK law. A sizeable proportion of these are thought to be polygamous.

Azad Chaiwala, 33, the Sunderland-based entrepreneur who developed the sites, is unapologetic and said that he wanted a polygamous relationship. His marriage was arranged by his family and his wife was informed from the start that she would not be the only one.

However, he said: “There was a taboo about the subject. No family member wanted to help me [find a second wife].”

Noting that there were sites catering to every “niche, desire, and fetish”, he said: “I think polygamy is more in tune with [man’s] nature than monogamy.”

While there is no reliable data on the number of polygamous marriages in the UK, experts believe that they are on the rise. That is the warning given by successive witnesses to the Commons home affairs committee, examining the issue as part of a broader inquiry into Sharia councils.

The law is the problem, they say. While polygamy is illegal, there is no requirement for a civil ceremony to precede a Sharia marriage. Imams are under no obligation to refuse a couple because the man is already married.

“The Marriage Act is so out of date,” Aina Khan, a leading family lawyer specialising in Islamic marriages, said. “It dates back to 1949 and only Anglicans, Jews and Quakers must register their religious ceremony with the register office . . . Now nearly three million Muslims, with origins from many countries, live here.” Young Muslims were driving the increase in polygamy, she said.

For men, polygamy offered more than one woman to have sex with and an easy route out of an unhappy marriage, she said, adding that having more than one wife was an outward sign of religiosity. Islam does not encourage polygamy but permits it in special circumstances such as if there is a shortage of men during a time of war.

She added: “It is sadly sometimes a woman who cannot easily find a husband of her own. Maybe she is older and is running out of time to have a child . . . Women who are well qualified and well off sometimes feel too good for the single men around and want someone of similar social standing, perhaps a wealthy businessman who is married.”

Of the couples who came to her seeking to part, a quarter involved polygamy issues, she said. “It is not a recipe for a happy marriage. They do not last.”

Splitting up can be financially devastating, as the unregistered marriage means that the woman has no financial protection under British law.