Ed Miliband has said the brutality of the Syrian civil war made him question his efforts to successfully block airstrikes against the Assad regime in 2013.
However the former Labour leader said he still concluded it was the "right thing to do".
Mr Miliband led a rebellion in 2013 against then-prime minister David Cameron's plans in response to chemical weapons attacks against civilians.
He was questioned over his stance on not intervening during an appearance on Channel 4 comedy show The Last Leg, hours after US President Donald Trump launched air strikes in Syria.
"The horrible scenes, ghastly scenes (of the conflict) obviously make me think about that," he said.
"I have thought a lot about it and whether it was the right thing to do but I think in the end, in my heart of hearts I do feel it was."
Mr Miliband said there was a lack of plan for the consequences of the strikes in 2013 and he feared Britain "being drawn into the Syrian civil war".
"The biggest lesson of the Iraq war was you don't send our forces into combat unless you know there is a clear plan," he said.
Boris Johnson looks "daft" and is in "deep political trouble" after pulling out of a meeting with the Russians, Alex Salmond has said.
The defence secretary condemned Vladimir Putin's Kremlin for its role in Bashar Assad's suspected chemical weapons attack on his own people.
The leading critic of the Trump administration has told ITV News the US and its allies must build on the president's justified airstrikes.