Downing Street has ruled out a second vote on military action in Syria, after pressure grew for David Cameron to return to the Commons with new evidence of chemical attacks.

Boris Johnson and others have suggested that MPs could be asked to vote again on military action in Syria as new facts emerge, despite suggestions it that a second defeat could fatally undermine the prime minister’s authority.

The development comes as:

• The US claims to have firm evidence of the use of sarin in Syria

• Parliamentarians react to reports that the UK nearly provided Syria with key components of chemical weapons

• The Arab league called for the UN to step into the conflict

Writing in his weekly column in the Telegraph, the London mayor said the pause the US has taken while Congress votes on military intervention on September 9th allowed time for new evidence to emerge which could firm up MPs’ response.

“If there is new and better evidence that inculpates [president Bashar al-] Assad, I see no reason why the government should not lay a new motion before parliament, inviting British participation – and then it is Ed Miliband, not David Cameron, who will face embarrassment,” he wrote.

“The Labour leader has been capering around pretending to have stopped an attack on Syria – when his real position has been more weaselly.

“If you add the Tories and Blairites together, there is a natural majority for a calibrated and limited response to a grotesque war crime.”

Shadow business secretary Chuka Umunna opened the door to Labour support for a second vote, saying that a “responsible opposition” would need to reconsider its approach if circumstances change.

Speaking this morning, the prime ministers spokesman said there were “absolutely no plans” for another vote and that Cameron believed “parliament has spoken”.

“I don’t think on any issue the government can go back to parliament every few days, or every week with the same proposition,” foreign secretary William Hague said.

“So on this particular issue that we voted for on Thursday; can we go back in the coming days and have that vote again? Well no, we can’t do that. parliament has spoken.”

Chancellor George Osborne said: “I don’t feel, frankly, more evidence or another week or more UN reports would have convinced them.”

Deputy prime minister Nick Clegg also effectively ruled out a second vote.

“We did it last Thursday, the answer was very clear, it was not the answer I was hoping for,” he said.

“I personally think there was a case for Britain, on humanitarian grounds, to participate in deterrent action to stop the further use of these abhorrent and illegal weapons but parliament did not agree and I think there is no point to keep asking the same question.”

But speaking in the Commons earlier, defence secretary Phillip Hammond appeared to buy himself some wriggle room by saying that there could be a second vote if “circumstances change very significantly”.

He added that it was “a bit rich” of Labour to ask for the conditions for a second vote, given it was their amendment last week which split the Commons and ruled out military intervention.

Shadow defence secretary Jim Murphy had earlier expressed unease about the outcome of the vote last Thursday.

“It’s not what I wanted,” he wrote on his website.

“I share the unease that we have gone from a stringent conditions-based approach to any UK military action to an unconditional policy of UK military inaction.”

Malcom Rifkind, chair of the joint intelligence committee, and Paddy Ashdown, former leader of the Liberal Democrats, have both said parliament should vote again if more evidence is discovered.

Meanwhile, questions were being asked about reports that Britain authorised export licences for chemicals which could be used in manufacturing chemical weapons, ten months after the start of the civil war in Syria.

The Department for Business authorised the sale of potassium fluoride and sodium fluoride, which were intended to be used for aluminium shower products and toothpaste, but EU sanctions were imposed before they could be delivered.

“There are very serious questions to answer as to why export licences for chemicals to Syria which could be used in the manufacture of chemical weapons were approved,” Chuka Umunna, Labour’s shadow business secretary, said.

“It will be a relief that the chemicals concerned were never actually delivered.

“But, in light of the fact the Assad regime had already been violently oppressing internal dissent for many months by the beginning of 2012 and the intelligence now indicates use of chemical weapons on multiple occasions, a full explanation is needed as to why the export of these chemicals was approved in the first place.”

Labour’s Thomas Docherty, who sits on the Commons Committee on Arms Export Controls, said he plans to table parliamentary questions on the issue.

“At best the government has been reckless and at worst negligent to permit the export of material that could have been used to create chemical weapons,” he said.

SNP Westminster leader Angus Robertson said: “This is utter hypocrisy from the UK government – deploring chemical weapons in public whilst approving the sale of items needed to make them.

“I will be raising this at Westminster as soon as possible to find out what examination the UK government made of where these chemicals were going, and what they were to be used for.”

Meanwhile, US secretary of state said hair and blood samples gathered after the chemical attack in Syria last month tested positive for sarin.

The findings come from the US’ own tests – not those currently being undertaken by UN weapon inspectors in Holland.

“In the last 24 hours, we have learned through samples that were provided to the United States that have now been tested from first responders in east Damascus and hair samples and blood samples have tested positive for signatures of Sarin,” Kerry said on NBC’s Meet The Press on Sunday.

“So this case is building and this case will build.”

The Arab League, which is holding a meeting of foreign ministers in Cairo, has called on the UN and the international community to take “deterrent” action against Syria.

It also called for those responsible for the chemical attack to face trial for war crimes.

But Jordan, a key ally of the US, has ruled out participating in military action.

“Syria is capable of facing up to any external aggression just as it faces up to internal aggression every day, in the form of terrorist groups and those that support them,” Syrian president Bashar al-Assad said on Sunday.