LONDON — An element of doubt Thursday crept into the British government’s ability to authorize military action on Syria.

British members of parliament are preparing for an emergency debate on the case for action in Syria, but the opposition Labour Party are insisting on seeing “compelling evidence” of the Syrian regime’s guilt over an alleged chemical attack before voting with Prime Minister David Cameron’s governing coalition.

Labour Party leader Ed Miliband said he was “determined we learn the lessons of the past, including (on) Iraq,” where evidence of of weapons of mass destruction was subsequently deemed to be false.

On Wednesday, Cameron reversed an earlier to decision to hold a single formal parliamentary vote that would specifically authorize British action prior to the conclusion of a report due from United Nations weapons inspectors.

Members of parliament will debate the motion Thursday, but a final vote from the United Kingdom on whether intervention should take place will now only come after U.N. investigators conclude their findings into the alleged chemical attack on Aug. 21 that may have killed hundreds.

Downing Street has said that the back down reflected the government’s “desire to proceed on a consensual basis,” but the move followed threats by the Labour Party not to support a possible strike. Labour are insisting that the U.N. inspectors be given more time to deliver their report to the U.N. Security Council.

A British draft resolution authorizing “necessary measures” to protect Syrian civilians was delivered to the Security Council in New York on Wednesday, where it was discussed.

Ahead of today’s parliamentary vote — due to take place around 10 p.m. local time (5 p.m. ET) — the British government received a letter from the Syrian government seeking a peaceful solution to the crisis.

“We implore you to communicate through civilized dialogue rather than a monologue of blood and fire,” the letter said, according to the BBC, who obtained a copy. The open letter was sent by the Syrian parliament speaker who also invited British MPs to send a delegation to the Mideast nation.

The U.N. inspection team will be leaving Syria by Saturday as its two-week mandate comes to an end, chief Ban Ki-moon said Thursday. Ban said in Vienna on Thursday that all opinions should be heard before anyone makes decisions on how to react to the alleged attacks.

President Obama said Wednesday he has concluded the Syrian regime is behind the attack. However, it’s not clear if the Western powers will wait for the U.N. experts’ findings before launching a possible punitive military strike.

U.S. officials, the Associated Press reported, say the intelligence linking the Syrian President Bashar Assad or his inner circle to the alleged chemical weapons attack that killed at least 100 people is no “slam dunk.”

The officials say questions remain about who controls some of Syria’s chemical weapons stores, and there are doubts about whether Assad himself ordered such a strike.

Both the U.S. and the U.K. are expected to reveal as early as Thursday evidence to support claims chemical weapons were used by Damascus.

ANALYSTS: Syria retaliation threat not empty words

Contributing: Associated Press