David Cameron is so "lame-assly dumb" that he will stumble into a second Scottish independence referendum, Jim Murphy has predicted.
The former Scottish Labour leader said the Prime Minister would accidentally give the SNP reason to hold another vote.
He also warned that Mr Cameron would not be trusted by Scots the second time around following his surprise push to strip Scottish MPs of voting rights in the wake of last year's referendum.
But he predicted that Scottish voters would reject separation again in a future vote.
Earlier, Mr Murphy had likened the SNP's general election campaign to "a quasi-religious rock concert" and hit out at what he said was a "post truth" atmosphere in politics north of the border.
The ex-MP resigned in the wake of Labour's disastrous general election performance in Scotland.
The party lost all but one of the 41 seats it had taken in 2010, while the SNP surged from 6 seats to 56.
Taking questions after a speech at the Policy Exchange think tank in London, Mr Murphy said: "There will be another referendum whenever the SNP can get away with it.
"Why wouldn't there? If you are an insurgent nationalist party with unprecedented power and with an absolute majority ... why wouldn't you try and engineer a set of circumstances to get you another referendum?
"My frustration is that Cameron is so lame-assly dumb on it that he is set to stumble into it and give them the excuse to do it."
He also warned the Conservative leader that if there was another referendum "the people of Scotland will ask 'where's the secret plan this time?'"
Mr Cameron had made a "dangerous" decision to prioritise party over country in the early hours of September 19, when he outlined plans for 'English votes for English laws' at Westminster, Mr Murphy said.
The Conservative Government says the reforms are necessary to protect the Union, in the light of growing concerns south of the border.
Mr Murphy also said that during the televised Scottish leaders' debates, he was taking part in a "a quasi-religious rock concert" where "no matter what truth you told it didn't really matter, in a post-truth type of argument".
On Labour's future he said he disagreed with calls for a 'trapdoor' to get rid of uninspiring leaders.
But in a warning shot to current leadership contestants he cautioned against believing Labour's mistake in Scotland had been to take part in the cross-party 'No' campaign.
Instead, he said, the problem had been the separate but lacklustre Labour campaign.
The issue is a live one as Labour looks ahead to the In/Out referendum on the UK's European Union membership.
Mr Murphy also condemned the description of Liz Kendall, one of those vying for his party's top job, as 'Taliban New Labour'.
Meanwhile, veteran left-winger Jeremy Corbyn said he would campaign on an anti-austerity platform after a last-minute surge in support saw him secure a place on the leadership ballot.
He will take on shadow health secretary Andy Burnham, shadow home secretary Yvette Cooper and shadow health minister Liz Kendall.
Contenders needed the 35 formal endorsements from other MPs to make the cut.
Mr Burnham received 68 nominations, Ms Cooper 59, Ms Kendall 41 and Mr Corbyn 36.
Mr Burnham used a speech in Crewe to suggest Labour could introduce a student loans-style system to help youngsters move around the country to take up technical apprenticeships.
Her also indicated that he would back changes to the tuition fees system for university students in England.
"We will also need to look again at how we open up university to more people," he said.
Ms Cooper called for the UK to invest 3 per cent of GDP on science, innovation and technology, warning that the country was being left behind by international competitors.
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