The Catholic church has launched a devastating onslaught on Labour, accusing the party of “violating moral law” and “losing ethical credibility”, as a new poll reveals the party is losing ground to the SNP in the Glasgow East by-election.

Bishop Joseph Devine, the second most senior Catholic churchman in Scotland, whose diocese covers the constituency, said Labour had lost the electorate’s trust and could no longer take Catholic support for granted. He claimed the party had “repudiated and abandoned Christian truths and values and sought to expel any notion of God from public debate and legislation”.

His intervention is potentially disastrous in a seat with a large number of Catholic voters. An ICM poll of Glasgow East voters, published today, reveals a 15% swing from Labour to the SNP since the last general election. Labour are on 47%, down 14 points from 2005, and the SNP are on 33%, up 16 points. The SNP require a 22% swing to win.

Labour’s own canvass returns reveal that, with 11 days to go until polling, its majority has been cut from 13,500 to 2,500.Fearful that losing the seat would spell the end of his premiership, Gordon Brown has ordered Labour MPs in Britain to campaign in the constituency. Party whips will ensure every minister and backbencher is seen pounding the streets.

However, with the party’s majority heavily dependent on the support of Catholic voters, Devine’s condemnation could not have come at a worse time.

Catholic leaders are furious at Labour MPs’ support for the Human Fertilisation and Embryology Bill (HFE), which will pave the way for research combining human and animal embryos, and at the party’s failure to back a lower time limit for abortions.

Last week the government postponed a parliamentary debate on the HFE bill, planned for tomorrow, amid fears that it could dent the Catholic vote in Glasgow East. The bill will not now complete its final stages until the autumn.

In a letter to Scottish Labour MPs, Devine said that while the government had won a Commons vote on the passage of the bill in May it had “in the process lost its ethical credibility in the nation at large”.

He added: “Christian people have not changed. It is Labour that has broken its pact with Christian voters. What are we to do when our religion is attacked and our conscience outraged? When one considers the self-inflicted injuries this Labour government has visited upon itself, one could be forgiven for thinking it had some kind of death wish.”

Senior Labour figures admit the seat could be lost to Alex Salmond’s resurgent nationalists. Defeat in the previously rock-solid constituency could shatter what little faith the party still has in Gordon Brown as an electoral asset.

David Marshall, who stepped down as the seat’s MP last month citing ill-health, took 60.7% of the vote in the 2005 election, winning with a majority of 13,507 over the SNP.

A significant amount of our vote is soft — it’s either reluctant Labour or it might not turn out because it’s not happy,” said a campaign insider. “I would be confident if the election was today, but there’s a whole fortnight to go and there are lots of factors outwith our control.”

Another senior Scottish Labour figure said the party’s campaign so far had been “a disaster” and feared it was likely to carry on in the same vein. “I think it’s 60/40 that we will win, that’s all,” the source said.

The campaign’s first weekend was mired in confusion after George Ryan, the local councillor expected to become Labour’s candidate, backed out of the selection meeting due to family pressures.

After a scramble to find a replacement, Margaret Curran was finally picked on Monday, but the following day falsely claimed to have lived and worked in Glasgow’s east end all her life. In fact, she has lived in the leafy southside of Glasgow, around Pollokshields and Langside, for more than 20 years.

Curran has also been accused of hypocrisy for planning to stay on as MSP for Glasgow Baillieston if elected as the MP for Glasgow East on July 24.

The Labour Party forbids such “dual-mandate” politicians. It had to suspend its own rules to allow Curran to stand and has repeatedly attacked Salmond for being both the MP for Banff and Buchan and MSP for Gordon.

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