MPs' refusal to condone military action against the Syrian government will cost Britain, Tony Blair has warned, in an attack which will only strengthen Labour dissent at Ed Miliband.
The Labour leader reportedly faces growing disquiet from within his own party as fresh images of atrocities by Bashar al-Assad's regime emerge from Syria.
Blair's article in the Sunday Times newspaper will only reinforce their concerns at the impact of the government defeat, which led Miliband to instantly seek - and get - reassurance from David Cameron that Britain would not pursue military action.
"Intervention can be uncertain, expensive and bloody. But history has taught us that inaction can merely postpone the reckoning," Blair warned.
"We haven't paid the bill for Syria yet. But we will."
The former prime minister, whose legacy of the Iraq war is widely blamed for MPs' unwillingness to back an intervention this time round, argued it was the fear of a 'quagmire' rather than controversy over the circumstances of going to war which had proved decisive.
Blair argued the Syrian conflict was part of a broader struggle in which Britain would eventually have to participate.
"At some point we will realise this is one battle, it is crucial to our security and we have to take sides," he added.
"The menace to our security is obscured by the complexity. But it is real... If Syria goes into the abyss, the consequences will not stop in Syria. Everyone, except us, is taking sides in Syria because they know what is at stake."
Blair's article does not mention Miliband but condemns the result of Thursday night's vote, which the Labour leader paved the way for by persuading his MPs to vote against the government, as "shocking".
Supporters of Miliband are delighted the defeat has moved the political narrative on from disquiet about his leadership, but he faces accusations of "playing politics" from a furious Downing Street.
Clashes between Cameron and Miliband's backroom staff began in the aftermath of their despatch box exchanges on Thursday.
They have continued this weekend, with No 10 sources quoted as accusing Miliband of "buggering around" by "playing politics" with a vote vital to national security.
Labour sources have insisted Cameron's "Flashman" character was ultimately responsible for the defeat, however.
The government lost Thursday night's vote by 285 votes to 272.