Ben Bradshaw said many Opposition MPs felt "uneasy" that Britain's options had been limited after the Government's shock defeat in the Commons last week.
Writing in the Guardian, the ex-culture secretary made clear he blamed David Cameron for "petulantly" ruling out direct intervention immediately after losing the vote.
But he also expressed alarm that Miliband was following the Prime Minister's lead rather than taking a "sensible and measured approach".
Calling for "urgent and deep reflection" by the party, the Exeter MP wrote: "Let's be clear what most Labour MPs were and were not voting for last week.
"We were not voting to support Britain taking part in immediate military action, but nor were we voting to rule it out completely. Neither were most Conservatives or Liberal Democrats.
"But because of Cameron's grotesque mishandling of the parliamentary recall and his 'rush to decision', that's where we are.
"All three main parties and their leaders ruling out any second vote whatever further war crimes Bashar al-Assad commits, however much worse the humanitarian crisis becomes and whatever the UN or the US and others decide to do.
"A considerable number of Labour MPs (many more than have so said so publicly) feel deeply uneasy about this.
"Where do we stand if Democrat America, Socialist France, Turkey and the Arab League go ahead with strikes?
"What is our view of how Britain should vote in the UN Security Council? How should we respond in the unlikely event that Putin was speaking the truth when he said he'd change his mind if it was proved Assad was responsible for the chemical attacks?
"Does being nice to Iran and talking about Britain's now diminished diplomatic and humanitarian role constitute a foreign policy?
"Labour needs to have clear and unambiguous answers to these questions. They will dominate politics through the party conference season."
Bradshaw said the amendment Labour tabled to the Government motion last week was "clear and sound".
It set out five conditions for military action - confirmation by the UN weapons inspectors that chemical weapons had been used, compelling evidence the regime was responsible, a UN Security Council vote, a legal basis, and clearly drawn objectives and time limits.
"Instead of sticking to this sensible and measured approach, we seem to have abandoned it because of the incompetence of a Tory prime minister," Bradshaw wrote. "We have allowed Labour foreign policy to be dictated by the Government."
The MP defended the principle of humanitarian intervention, pointing to Labour's successes in Kosovo and Sierra Leone, and cautioned against seeing everything through the "prism of Iraq".
"Leadership is about learning the right, not the wrong, lessons from the past, judging each case on its merits and deciding accordingly," he insisted.
"Ed Miliband was absolutely right to put a brake on Cameron's 'rush to war', and was as taken aback as we all were when the prime minister, at his petulant worst, took the military option off the table following his defeat.
"Miliband now needs to use the extra time offered by the UN process, the G20 and the US congressional debate to reflect on Labour's position and discuss it with colleagues."