Catholic schools in Wales may have broke the law when they read out a anti-gay marriage letter from a religious education group to pupils as young as 11, the government in Cardiff has said.

The Welsh administration said on Tuesday it did not fault the Catholic Education Service (CES), which wrote to 359 state schools in England and Wales earlier this year to promote its view of marriage as between one man and one woman, but the schools that read it out.

Leighton Andrews, the Welsh minister for education and skills, said not providing a converse view to students could be seen as a political act.

In a written statement published on Tuesdayhe said the letter did not breach the Equalities Act as it did not contain homophobic statements, but reading it to students was in breach of a separate law that prohibited the promotion of partisan political activities.

"Section 406 of the 1996 Education Act Act requires that local authorities, head teachers and governing bodies shall forbid the pursuit of partisan political activities by any registered pupil who has not attained the age of 12, and forbids the promotion of partisan political views in the teaching of any subject at the school," he said.

"While Section 407 of the 1996 Act requires schools to take whatever reasonably practical steps are necessary to ensure that where political issues are brought to the attention of pupils they are offered a balanced presentation of opposing views."

The CES was encouraged by the Catholic Church to forward the text written by Archbishops Vincent Nichols and Peter Smith which was read out during masses in March.

In one Catholic school, pupils claimed their head teacher put together a presentation which ended with the words: "Sign the petition".

According to a spokeswoman from the CES, the letter was distributed to schools - which were allowed to approach the letter in a way they thought was "appropriate".

"We asked schools to draw pupils' attention to the petition, which simply affirms Catholic beliefs," she told The Huffington Post UK.

Andrew has written to all Roman Catholic secondary schools in Wales reminding head teachers and governing bodies of their "duty and responsibilities under the 1996 Act" and has asked them to ensure that pupils be made aware of arguments in favour of gay marriage in order to give them a balanced perspective.

Chief executive of gay-rights campaigners Stonewall, Ben Summerskill, said the schools had engaged in a "shocking breach" of their duty of care.

"It categorically shouldn’t be involved in such a live political issue, particularly in a way that demeans gay pupils," he said.

"We’re sure students and parents will be upset that this school is creating deeply unpleasant environment for gay young people when so many other faith schools are doing a brilliant job tackling homophobia."

The coalition government is expected to formally announce plans to introduce gay marriage when it sets out its legislative agenda for the coming parliament tomorrow.

While the plans have the backing of the leadership of all three main political parties in Westminster, the government has faced fierce opposition from some religious groups and backbench Tory MPs.