All heterosexual couples in England and Wales will be offered the chance to “convert” their marriages to civil partnerships, or vice versa, under plans being considered by ministers.

The Government Equalities Office (GEO) is launching a consultation on how to fulfil its pledge to extend civil partnerships to opposite sex couples.

Because those who are already married never had a chance to opt for a partnership, there is likely to be a limited period of a year of more when couples can choose to reconfigure their legal relationships. There would be a fee for the process.

Last year the prime minister, Theresa May, promised to offer every couple in future the choice between a civil partnership and marriage when they formalise their relationship.

Her decision followed a government defeat at the supreme court where judges unanimously found that it was discriminatory to restrict civil partnerships to gay couples.

They ruled that the refusal to allow the equality campaigners Rebecca Steinfeld and Charles Keidanto form a civil partnership was a breach of their human rights.

The couple are still waiting to enter into a civil partnership. The GEO hopes the first opposite sex partnership ceremonies will be held before the end of the year.

The new legislation, the GEO says, “will allow churches and religious organisations to choose whether to host civil partnerships only for same-sex couples, only for opposite-sex couples, for both, or not to host civil partnerships at all.

The consultation will seek public opinion “on giving opposite-sex couples an opportunity to convert their marriage into a civil partnership, and then bringing all conversion rights to an end”, the GEO said.

Providing a period for “conversion” will allow opposite-sex couples the opportunity to enter into a legal relationship that was not previously available to them. During the conversion period, same sex couples will also be allowed to transform their civil partnerships into more traditional marriages.

Penny Mordaunt, the minister for Women and Equalities, said: “There are all sorts of reasons why people may choose not to marry, but for a long time it has been the only option for many wanting the legal security it provides.

“Last year the prime minister announced [the] government would support the extension of civil partnerships to opposite-sex couples. This is a fantastic step, providing an alternative to marriage for these couples.

“We must now consider those who didn’t haven’t had this as an option previously, that’s why we’re consulting on whether opposite-sex married couples can convert their marriages to civil partnerships.”

Civil partnerships have been available to same-sex couples since 2005. The Marriage (Same Sex Couples) Act 2013 then allowed same-sex couples to enter a marriage or convert their civil partnership into a marriage.

The only jurisdiction in the British Isles that permits opposite-sex civil partnerships is the Isle of Man. Some couples have travelled there for ceremonies, but the unions are not yet recognised in the UK. That may change under the proposed legislation.

Other countries such as South Africa, New Zealand and the Netherlands allow couples to choose either civil partnership or marriage.

Many family rights campaigners hope that opening up civil partnerships will encourage many of the 3.3 million unmarried couples in England and Wales to formalise their relationships and obtain the legal rights available in marriage.

The Marriage Foundation supports extending civil partnerships to heterosexual couples, saying they are “infinitely preferable to unthinking and risky cohabitation”.