Britain has landed itself in a "constitutional mess" in the wake of the summer vote against military action in Syria, in which the Commons can be guaranteed to back intervention only to defend the Falkland Islands and Gibraltar, according to a former Foreign Office minister.

In his first interview on the Syrian crisis since losing his ministerial post in the autumn reshuffle, Alistair Burt said the failure of MPs in August to back the principle of military action against the Assad regime for the use of chemical weapons had left the mainstream opposition forces "absolutely devastated".

Burt, who was the minister overseeing Syria policy for three years, said the rules on the power of parliament to sanction British military action were evolving and required clarity. He revealed that ministers were unclear after the Syria vote whether the British government was even permitted to provide intelligence or logistic back-up to a US strike designed to deter President Bashar al-Assad from using chemical weapons.

"There is now a question mark about what parliament actually will authorise military support for," Burt said. "There is the Gibraltar and the Falklands – I think we can assume those. I am not sure we can assume anything else. Where does that leave us and our partnerships around the world? We have put ourselves in a constitutional mess this way."

Sir Malcolm Rifkind, the former Conservative foreign secretary, also suggested that Assad was strengthening his grip on power after the failure of the international community to support mainstream opposition forces, which have been undermined by internal weaknesses.

But Rifkind, a supporter of a strike against Assad for using chemical weapons, added that in retrospect the Commons vote had strengthened diplomacy in the Middle East. He described the sequence of events as "a chapter of accidents which, for once, had a happy outcome, and which could have equally had a very unhappy outcome".

Rifkind said: "In the last three or four months we have had, in a way no one predicted, not one but two diplomatic breakthroughs: the Syrian chemical weapons agreement and the interim deal in Iran. Don't underestimate the implications of that. Suddenly diplomacy has got a good name. The international community, in quite different ways, have got their act together and in two of the most intractable problems in the Middle East."

The remarks by Burt and Rifkind were made in interviews with the Guardian examining the events of late August in which David Cameron tried – and failed – to secure parliamentary backing for the principle of military action against the Assad regime in response to a chemical weapons attack on a Damascus suburb.

The vote was hailed by some as a reassertion of parliament. The Labour party leader, Ed Miliband, argued that an "ill-thought-out" intervention would make the situation worse for the Syrian people.

After the vote, Cameron surprised MPs by immediately ruling out Britain joining military action at any time. "I strongly believe in the need for a tough response to the use of chemical weapons, but I also believe in respecting the will of this House of Commons … the British parliament, reflecting the views of the British people, does not want to see British military action. I get that, and the government will act accordingly."

The defeat shaped events well beyond Westminster by providing what one senior diplomat called a "get out of jail card" for Barack Obama, who backed away from military action, creating a space for the deal between Washington and Moscow to rid Syria of chemical weapons.

The interviews also revealed:

• Continued resentment in Downing Street over the conduct of the White House, with No 10 believing Cameron worked hard to persuade President Obama to stand up to Assad, only to see military action collapse after officials felt they were bounced by the White House, leaving the prime minister no time to prepare his own side.

• Senior British foreign policy figures say Assad is now in danger of winning, and the failure to send earlier clear signals to Assad may mean it is too late to save the Syrian opposition.

• Tory whips were told middle-ranking ministers and parliamentary private secretaries would have resigned if Cameron had tabled an amendment immediately endorsing war. On the afternoon of the vote, Tory whips were seeking a deal with Labour to abstain.

• Ed Miliband had a disagreement with the shadow defence minister, Jim Murphy, on the morning of the parliamentary votes on 29 August when the Labour leader told him the party would vote against a government motion even after Cameron agreed to his demand to delay a vote authorising military action until a later date. Murphy was later demoted from his post in what was described as a "punishment beating".

Douglas Alexander, the shadow foreign secretary, has said the Labour decision to oppose a rush to war was vindicated by events, including the agreement to remove Assad's chemical weapons and the possibility of a diplomatic breakthrough in talks in Geneva.

But Burt – a close ally of the foreign secretary, William Hague – revealed his deep anger at the failure of MPs in August to back the principle of military action. "We have put ourselves in a constitutional mess this way. I think government needs to take executive action in foreign affairs. It informs parliament. If parliament does not ultimately go for it, then the issue becomes a vote of confidence issue. I don't think you can handle foreign affairs by having to try to convince 326 people [a majority of MPs] each time you need to take a difficult decision. You do it and if they don't like it, they can vote you out and they can have a general election."

Burt insisted the British government "knew exactly what would happen if there was not a strike against Assad over chemical weapons. He goes on. And the only thing that would deflect this man and this regime is if they fear they are going to end up in a storm drain with a bayonet up their backside. If they don't fear that, they will go on killing as many people as they need to stay in power."

Burt added: "I think moderate Syrian opposition was absolutely devastated by this decision. They felt in a sense that their last hope had gone. They had heard the west talk a good game about support. They had heard Americans hint about arming them in some way or other. They had heard us say we support you 100%, but then our parliament will not let us give you anything."

He also claimed the failure to strike against Assad strengthened the previously small extremist Syrian opposition. "We have no idea what the serious extremists' numbers are, but I suspect they are small," he said. "The Free Syrian Army still command a lot of support, but they don't have sufficient military means to do it. If you are fighting to remove Assad and you want to stay alive to see a new dawn, you will fight with the people that will give you the best chance of survival rather than martyrdom."

One senior Foreign Office source echoed the concerns of Burt and voiced the hope that Britain would not lose its ability to act diplomatically and militarily. The source told the Guardian: "It sounds like a cliche, but, on the whole, the UK does punch above its weight in international affairs. Two key reasons for our diplomatic strength are, first, our status as one of the five permanent members of the UN security council and how we use that position; and, second, our ability to be more flexible, adaptable and nimble than others – both diplomatically and militarily.

"I really hope that the lesson from August's parliamentary vote is that however we take decisions about military action in the future, we do so in a way that preserves rather than constrains our comparative advantage [or our ability to be nimble]."

As the political world prepares for a year of commemorations marking the start of the first world war, Rifkind said the chemical weapons deal on Syria showed that a series of accidents did not have to lead to war.

"You murder an archduke and something bad happens and because that has happened something even worse happens and then we end up with the first world war. The opposite can also potentially happen – having got that initial breakthrough on Syria, the Russians and the Americans both benefited from it."

Burt doubts, however, that the Commons vote changed the American calculations, saying that Obama could easily have worked instead with the French if he had wanted to pursue military action. "If Obama really wanted to do what he should have done, it would have been very nice for America to have us, but the reality was going to be 'François [Hollande] come in, Dave [Cameron] I am busy.' I don't really buy this argument by some of my colleagues that the Commons vote persuaded the United States not to enforce a chemical weapons convention against a state that had tried to gas its own people, and that Obama was saying 'I cannot do this because the British parliament has not supported a vague motion'. I do not get that."