The religious faith of Scottish Labour leadership candidate Jim Murphy, has become central to an extraordinary row between the Free Church of Scotland and the Scottish Secular Society (SSS).

The church, historically not a close ally of Roman Catholicism, has accused the SSS of sectarian behaviour after the group made repeated remarks about Mr Murphy's RC faith.

The moderator-elect of the Free Church Rev David Robertson claimed in his blog that the Labour MP for East Renfrewshire had been continually targeted by SSS simply for being a Roman Catholic.

He said SSS founder and media spokesman Garry Otton recently posted four different threads about Mr Murphy over a 48-hour period, all making reference to his religion and support for denominational schools. Mr Robertson described some of the comments as "disturbing".

The minister of St Peter's Free Church in Dundee, said: "The Scottish Secular Society have posted several stories about 'Catholic fanatic/extremist/Pope Benedict fan' Jim Murphy over the past few days.

"I find it particularly disturbing this constant referral to Jim Murphy as Roman Catholic - what does that have to do with anything? It comes worryingly close to the kind of anti-Catholic sectarianism that plagued the West of Scotland - perhaps it still does.

"It is of no relevance or interest to me that a particular political candidate is Roman Catholic or not. Mr Murphy should be judged on his political views and abilities, not what church he belongs to. It is ironic that of all groups the Scottish Secular Society continues to highlight religious affiliation as though this were somehow a disqualifying factor.

"It does not bode well for freedom of religion in the 'secular' society they are striving to create."

Mr Robertson said some people were keen to use secularism "as a guise for their militant atheist, anti-theist or more often just anti-Christian agenda."

But Mr Otton insisted that the SSS had no problem with Mr Murphy's beliefs, "but a very great problem with the way in which we fear they will influence his political decisions.

"In particular, we don't approve of support for the idea that bishops can be put in charge of sex education in Catholic schools. We are also concerned that he will defend privileges for organised religion, segregating children on the basis of their parents' religion in denominational schools with separate staff rooms and entrances. We are utterly opposed to sectarianism in any shape or form. "

He said Mr Murphy had been reported as praising the US because religion had a bigger role in politics. "That is not a scenario the Scottish Secular Society would welcome in Scotland."

He said there was also general agreement amongst secularists that unelected religious representatives, both Catholic and Church of Scotland, voting on how councils should deploy their limited education budgets was absurd.

"The Scottish Secular Society is the appropriate organisation to challenge the religious privileges," he said.

Jim Murphy said of this exchange of views:

"The days of judging someone by what Church they are a member of or what school they went to must surely come to an end in Scotland. Religious discrimination is as stupid as it is sickening. In the modern Scotland we like to pride ourselves on being a tolerant people, but a minority seem trapped in the past and fixated by other people's faith.

"I believe that people should be allowed to celebrate their faith free from harassment and that those who don't follow any religion or religious faith should also have that right to express their views openly. No-one should be singled out because of their attitude to faith."