STRAIGHT couples would be allowed to enter civil partnerships under plans being drawn up by SNP ministers who are to separately outline proposals next week to legalise same-sex marriage.

The move follows a campaign for heterosexuals to be offered an alternative to what some people regard as the old-fashioned or sexist institution of marriage.

While the proposal will not be included in the forthcoming gay marriage bill at Holyrood, ministers will pledge to assess male and female civil partnerships with a view to future legislation.

Currently, men and women in relationships cannot enter civil partnerships, which were created to give gay and lesbian couples the same legal rights, if not the status, of marriage.

As is the case with gay couples, heterosexual couples who enter into a civil partnership would have the same legal rights as if they were married.

A spokesman for the pressure group Equality Network, which has campaigned in favour of gay marriage and straight civil partnerships, said opening up the idea of civil partnerships to all was widely supported.

Tom French said: "Opening up civil partnership to mixed-sex couples has always been a key aim of the equal marriage campaign in Scotland and is supported by the majority of Scots.

"The Equality Network believes the law should be equal with the same choices for everyone. We hope the Scottish Government will set out a clear path to changing the law in this area."

Civil partnerships between heterosexuals are under consideration in England, although concerns over extra pension costs have been raised.

SNP ministers have been discussing possible changes to civil partnerships with opposition MSPs and interested parties in the run-up to publishing the gay marriage bill, which is expected on Tuesday.

A source said: "There is a growing consensus around allowing mixed-sex couples to enter civil partnerships. It would be too complicated to achieve by an amendment to the bill but there is a commitment to take it forward."

Mr French said equality was needed for those unconvinced by the idea of a traditional marriage.

He said: "Some people do not believe in the institution of marriage, either because they believe it is too patriarchal or too closely related to religion.

"Countries which allow a form a civil partnership for all have seen an increase in the number of people in registered partnerships. If one of the Government's objectives is to increase the number of people in stable relationships, this would be a good move."

He pointed to France, where formalised alternatives to marriage are available to both gay and heterosexuals and the number of couples tying the knot in such circumstances has risen.

Equality Network found in a poll last year that 71% backed the idea of straight civil partnerships, compared to 64% for same-sex marriage.

A spokesman for Scotland for Marriage, which is opposed to gay weddings, said: "This won't redefine marriage; it will wreck it. It will introduce a marriage-lite alternative for straight couples who don't want to get hitched, giving them all of the rights of marriage with none of the matrimonial responsibilities.

"The Westminster pensions minister looked into extending civil partnerships to straight couples in England, and said it will cost the taxpayer £4 billion in public pension rights alone."

Health Secretary Alex Neil signalled last month legislation allowing same sex marriage would be presented to parliament before the summer recess.

The main party leaders support the move and ministers believe Scotland's first gay wedding will take place in 2015. However, the bill faces opposition from the Kirk, the Catholic Church and Muslim leaders.

Scotland for Marriage is urging opponents to lobby 50 MSPs in areas it believes are unsympathetic to the idea.