Tony Blair has warned of the dangers of leaving the political centre ground, in comments the Conservatives seized on as a veiled criticism of Ed Miliband.
In a wide-ranging interview, the former Prime Minister also urged his party to stand up for the UK's membership of the EU.
Mr Blair won three elections as Labour leader, two of them in landslide victories.
Next May, however, Labour faces a tough contest with the Conservatives for the keys to Downing Street.
Earlier this month Mr Miliband was forced to hit back at reports some Labour MPs wanted him to stand aside before the general election.
Labour was boosted by a poll bounce this weekend - despite the row over an allegedly "sneering" tweet about white vans and England flags that led to the resignation a frontbench MP.
In Scotland, however, surveys suggest Labour could still lose a number of seats to the SNP next May.
Another poll, by the Tory peer Lord Ashcroft will today show UKIP second in Doncaster North, suggesting that if Tory supporters vote tactically for the eurosceptics Mr Miliband could lose his seat.
In the interview, Mr Blair hinted that could he expand on his opinion of Mr Miliband at a future date.
He said that the way for parties to win a "substantial majority" in 2015 was to have a "strong political lead" in the centre ground, combining "the politics of aspiration with the politics of compassion".
He also suggested that a Labour majority was still possible, despite what he described as a "lot more fragmentation" which has seen parties like the SNP, the Greens and UKIP gain political ground.
Conservative Vice-Chairman Bob Neill MP said: "For all his many faults Tony Blair was still Labour's most electorally successful leader, but even he thinks Ed Miliband's just not up to it.
"People realise that Ed Miliband is a weak leader whose policies of more wasteful spending and higher taxes don't add up to a credible economic plan."
Mr Blair also warned that to leave the European Union would be viewed as an "extraordinary act of self-destruction" by the UK.
There was a "high degree of alarm" internationally about the possibility of a UK exit from the EU, he added.
But he warned Labour not to try to "out-Ukip" Nigel Farage's party.
To tackle the eurosceptics, Labour had to "analyse their policies, show how destructive they would be" and set out positive alternatives "rather than joining in selling people a false and illusory elixir of hope, which is around if you stop more Polish people coming to Britain you're going to provide more jobs in the poorest communities in the UK," he said. His party "would not lose" by standing up for UK membership of the EU, he added.
A poll at the weekend suggested that Mr Miliband was fully trusted by just 2% of Scottish voters. The survey of more than 1,000 adults by Survation put the SNP on 45.8%, with Labour on 23.9%.
A poll earlier this month suggested that Labour could lose all but four of its seats in Scotland in May, dealing a significant blow to Mr Miliband's hopes of becoming Prime Minister.
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