The leader of the Roman Catholic church has said that politicians have a moral duty to vote against gay marriage or partnership legislation.

In a 140-page document covering a range of moral issues, Pope Benedict has reiterated his hard-line attitude towards gay people.

The Apostolic Exhortation, published today, calls on all Catholics to uphold what the Pontiff refers to as fundamental values.

Among those values he reiterated his view that marriage is the institution on which society is based, and can only be between a man and a woman.

“Catholic politicians and legislators, conscious of their grave responsibility before society, must feel particularly bound, on the basis of a properly formed conscience, to introduce laws inspired by values grounded in human nature,” he wrote, according to Reuters.

The Pope said it was up to individual bishops whether to deny the sacrament of communion to politicians who defy the church’s teaching.

In the UK, the role of Roman Catholic politicians has been under scrutiny since Ruth Kelly was appointed as Communities Secretary last year.

Ms Kelly, 38, is a devout Catholic and a member of Opus Dei.

She was a controversial choice as Communities Secretary, especially as the department was given responsibility for gay rights.

Ms Kelly has refused to answer questions about her personal views on homosexuality, but told PinkNews.co.uk in May 2006:

“People should be allowed to decide how they live their lives. I believe in a tolerant, diverse, multicultural society where everyone is protected from discrimination.

“I will fight discrimination, whether it be on the grounds of race, gender, disability or sexual orientation.”

Ms Kelly was the focus of much of the criticism the government faced after it was revealed that they considered granting exemptions to anti-discrimination legislation to Roman Catholic adoption agencies.

It was widely reported that Ms Kelly was the driving force behind the exemptions, which would have allowed Catholic agencies to continue to refuse to consider gay and lesbian couples as adoptive parents.

It later emerged that it was in fact the Prime Minister, Tony Blair, who wanted to grant an exemption Roman Catholic agencies from the Sexual Orientation Regulations.

The SORs come before parliament next month. Mr Blair’s wife Cherie is a devout Roman Catholic and the Prime Minister regularly attends Mass with her.

In the end Roman Catholic-run adoption agencies have been given a “grace period,” until the end of 2008, to comply with the regulations, which are due to become law at the end of April.

Stonewall, who campaign for gay equality in the UK, said of Ms Kelly that while the religious views of politicians are a private matter, government ministers must deliver policies without fear or favour.

Speaking about the Pope’s Apostolic Exhortation, Alan Wardle, Stonewall’s director of parliamentary and public affairs, told PinkNews.co.uk:

“Politicians by all means can listen to the Pope but at the end of the day it is not the Pope who votes for them in parliament.”

The Pontiff’s comments come as Italy’s government tries to find a compromise on gay partnership rights.

Many other Catholic countries, such as Ireland and Mexico, are currently making moves towards gay equality in the form of partnerships.