Shadow foreign secretary Douglas Alexander said he had "very serious doubts" about William Hague's decision to support lifting the European arms embargo on Syrian rebels.
He said the Foreign Secretary's support for opening up the possibility of supplying weapons to Syrian opposition groups risked jeopardising a proposed peace conference in Geneva.
Mr Hague has insisted here are no current plans to arm the rebels and that the lifting of the European Union embargo was a bid to increase the pressure on all sides to attend the peace talks.
But Mr Alexander described the country as already being "awash" with weapons and said the prospect of the rebels being armed meant the incentives to attend the peace conference were "pointing in the wrong direction".
He told BBC1's Andrew Marr Show: "I think Western European politicians have a responsibility to be clear with the rebels that actually the Geneva two (peace) process is the way forward.
"And if we are saying listen, if you don't turn up at these talks there is the prospect that Europe will provide you with arms, I think the risk is that the incentives are pointing in the wrong direction."
He went on: "I think there are very serious doubts about the judgment that he (Mr Hague) exercised last week along with the French in demanding the lifting of the EU arms embargo.
"It was suggested that somehow Europe agreed to this arms embargo - it required unanimity, and basically the British and the French said notwithstanding the opposition of the rest of Europe we are simply not going to accept the arms embargo continues after this weekend.
"I fear that with the best of motives he's ended up in a situation where the peace talks may not happen and we all want to see that happen I'm sure, including the British Government."
Mr Alexander said the priority should be unifying the disparate rebel forces, rather than arming them, and said the main opposition group, the Syrian National Coalition, had been "completely incoherent" in recent months.