The government's power to launch military action is now "in a mess" and the requirement to seek prior approval of MPs should be abandoned, the former Middle East minister, Alistair Burt, says.

Burt's remarks come as Downing Street becomes increasingly concerned that Bashar al-Assad, the president of Syria, is trying to avoid the commitment to destroy chemical weapons, after striking a deal with the Russians last summer when faced by the possibility of US military strikes over weapons use.

When Burt was Middle East minister he was deeply involved in the failed attempt to persuade MPs to endorse the option of UK military action to punish Assad, but writing in the Guardian he now opposes the requirement to stage a prior vote.

Burt, who was dismissed in the summer reshuffle but is widely respected on the Tory benches, asks: "If we are now in the position of having to convince half of parliament plus one before difficult foreign policy executive action can be taken, to what can government commit itself in discussions with allies, or preparation in advance for regional strategic defence?"

Burt is one of several defence and foreign specialists concerned that Britain, building on the parliamentary vote called by Tony Blair for military action in Iraq in 2003, has adopted a new constitutional position where it is unclear whether any military action can be taken without prior approval of MPs – a position radically reducing the authority of diplomats and weakening the efficiency of the UK military.

The Tory MP said: "The democratic balance will not be unfairly skewed by the executive retaining power to act solely by consultation and not immediate vote."

He says he fears the British public would not support military action abroad – save to protect the Falklands and Gibraltar – but claims: "Just occasionally politicians need space and time to take unpopular action which they believe in the long run is in their nation's interest".

He says: "Provided they are ultimately accountable to their public through election, or that a vote of confidence could do for a government's existence, I do not believe the democratic balance is unfairly skewed."

He suggests that, unwittingly, Britain has got itself into a position where backbenchers decide whether the country can go to the aid of a Gulf state friendly to the UK.

Gordon Brown and Jack Straw, who served as foreign secretary under Blair, had tried to codify the war-making powers of parliament, but the proposal has largely ended with a series of unwritten conventions.

Britain, for instance, did have a vote on Libya, but not for the logistical and other support the UK gave the French for their actions in Mali.

At prime minister's questions on Wednesday, David Cameron for the first time admitted there were doubts that Assad might not adhere to the chemical weapons agreement.

There have been growing signs that he is evading the UN weapons inspectors, an issue Cameron raised with Putin directly by phone this week.

Cameron told MPs: "After a very promising start, with chemicals not only being discovered and removed but destroyed, there seem to be indications that the programme is now slowing and that not all the necessary information is forthcoming".