The co-convenor of the Scottish Green Party Patrick Harvie MSP has challenged the Scottish Catholic Church to produce evidence for its claim that gay relationships harm life expectancy.

The Catholic Church has said it will respond later this week.

Appearing on Scotland Tonight last week to discuss the Scottish government’s plans to legislate for same-sex marriage, spokesman Peter Kearney said: “There is a link between same-sex sexual practice and early death.

“That’s not something that the Catholic Church believes; there is an overwhelming body of medical evidence to suggest that. One study has shown that the life expectancy of a practising homosexual man will be reduced by something between 12 and 20 years.

“We only need to imagine the complex infections, diseases and illnesses that are caused. I think we’re all aware of it. We tend to indulge ourselves in a willful fantasy that there are no dangers, that it’s not harmful.

“That’s not a particularly compassionate response for a society to take.”

Now, Scottish Green Party co-convenor Patrick Harvie has called for Mr Kearney to substantiate his claims.

In an open letter to the Catholic spokesman, Mr Harvie writes of the suggestions: “Whether this line of argument has any bearing on the same sex marriage debate is unclear; I am sure you were not implying that poor health should be a legal barrier to marriage or civil partnership for anyone, regardless of their sexuality.

“However it is important that those of us in the privileged and powerful position of speaking on these issues in the national media don’t confuse proper scientific evidence with some of the distortions which circulate online or in the wilder imaginations of some campaigners in the very polarised debate in the US.

“I am sure that you will be aware of some of the studies which have been misused in this way. The work of the avowedly anti-gay campaigner Paul Cameron for example, has been thoroughly discredited by the American and Canadian Psychological Associations and by the American Sociological Association and although it is based merely on a sampling of obituaries in gay newspapers it continues to be cited by some campaigners as though it is based on robust science.

“Similarly, research by Hogg et al published in the International Journal of Epidemiology (which was designed to make an assessment from limited available data about the impact of HIV in urban Vancouver in the days prior to modern antiretroviral therapy) has been misused to such an extent that the authors have had to issue a statement to clarify the actual meaning of their work and to oppose “the use of our research in a manner that restricts the political or human rights of gay and bisexual men or any other group”.”

Mr Harvie, who worked in HIV prevention and equality campaigns at the Gay Men’s Project at PHACE Scotland before his election added: “I very much hope that you have not mistaken such distortions of science for the real thing, or worse still decided to repeat the deliberate distortions and untruths being peddled by certain campaigners in the US. I would therefore like to offer you the opportunity to clarify your comments from the programme, and to give clear references to the “vast array”, or “overwhelming body of medical evidence” to which you referred.”

PinkNews.co.uk first asked the Catholic Media Office for clarification on its use of the studies mentioned above in December 2011 when Mr Kearney cited them in support of the position.

Today, Mr Kearney said he would be responding to Mr Harvie later this week and stressed a distinction between gay relationships and gay sexual activity.

The Catholic Church in Scotland has facing criticism after footage surfaced of the Archbishop-elect of Glasgow Philip Tartaglia drawing a link between homosexuality and the early death of a gay Catholic MP last year.

A charity, the David Cairns Foundation, named for the MP and set up after his death said the Catholic Church had made “scurrilous” and “hurtful insinuations”.

It said: “David died in tragic circumstances due to complications arising from acute pancreatitis. A gallstone blocked his pancreatic tract leading to further infection acquired during the two months he spent in hospital prior to his death.

“To suggest that his death was in some way connected to his sexuality is totally erroneous and made purely for political gain to somehow influence the debate on Equal Marriage.

“We take exception to the comments made by both Bishop Tartaglia, subsequently re-enforced by Mr Kearney in his television interview. Despite the half- hearted apology offered by the Bishop on Tuesday evening, Mr Kearney continued to support the Bishop’s comments in a manner that we and many others would consider to be homophobic.

“Whilst we do not wish to stifle the debate in Scotland and the rest of the UK concerning Equal Marriage, we would implore leaders of faith groups to use language more befitting their office. We can state that a more considered and pastoral approach from both the Bishop and Mr Kearney would likely not have caused the injury felt by David’s partner, friends and family.”

Jack Drescher, a member of the World Health Organisation’s Working Group on the Classification of Sexual Disorders and Sexual Health told the Scotsman: “There are no reputable scientific studies that show gay men, in general, die earlier than heterosexual men. Unfortunately, there are individuals known to oppose gay civil rights who create their own ‘data’ and reach questionable conclusions about gay men’s lifespans.

“Although these ‘studies’, more like pseudo-studies, have little or no basis in empirical science, some choose to publicise them to the general public and use them as political scare tactics.”