Defence secretary Philip Hammond has slammed the UK government plans for equal marriage, saying religious groups will be at risk.

With the UK government set to publish the marriage equality bill tomorrow (25 January) and a free vote to come later this year, the senior Cabinet Conservative minister has stated his firm opposition to the bill.

In a letter to a student constituent, Hammond challenged the 'robustness' of 'protections for religious organizations' contained within the bill.

The MP for Runnymede and Weybridge in Surrey added civil partnerships had 'removed the elements of practical discrimination' which used to exist against same-sex couples.

Hammond had previously blasted the government's equal marriage plans, describing them as 'too controversial'.

Royal Holloway students will protest at the University of London in Egham on Friday (25 January) where Hammond is due to speak.

Protesters will display banners and have passed a motion to lobby the MP. A gay 'kiss-in' has also been suggested.

The equal marriage bill applies a 'quadruple lock' of protections for religious organisations which do not wish to conduct gay marriage ceremonies.

It will be illegal for the Churches of England and Wales to perform same-sex marriages. The Equality Act 2010 will also be amended to prevent discrimination claims.

Other religious groups such as Liberal Judaism, Unitarians and Quakers, who are in favor of gay marriage, will be allowed to opt in if they wish.

Despite the coalition government's official line on equal marriage, other top ministers have come out against it.

Environment secretary Owen Paterson and Welsh secretary David Jones have said they will vote against the bill.

Communities minister Baroness Warsi has also been reported as having raised concerns about equal marriage.

Speaking to Gay Star News, a Conservative Party spokesman said: 'We believe marriage binds us together, brings long term commitment and stability, and makes society stronger.

'These proposals mean marriage would be available to all couples who wish to show their commitment to each other.'