UK military adventures should not require advance approval from MPs.

Does the prime minister need a vote in the House of Commons before authorising military action abroad? It certainly looked that way from David Cameron’s decision to recall MPs for a day-long debate in the middle of their party conference season. But according to one particularly thoughtful Conservative MP, Cameron’s decision to seek Commons approval for UK air strikes on Islamic State extremists in Iraq was a ‘serious mistake’.

We are not talking here about whether military intervention is desirable. I am sure it is, even if its effect proves largely symbolic.

And we are not talking here about whether military intervention is legal. The Commons motion, approved by 524 votes to 43, made it very clear that the UK was taking action in Iraq at the request of the Iraqi government; any extension to Syria would require a separate vote. Cameron published what amounted to a summary of the legal advice he had received from the attorney general, Jeremy Wright, which said the Iraqi government’s request for assistance provided a clear legal basis for intervention. Nobody disagrees.

Until recently, ministers would not say whether they had even taken advice from the law officers. That was consistent with lawyer-client privilege. But it was also a convention of the constitution.

A convention, in this sense, is not a meeting or a conference – even though Ed Miliband was calling for a ‘constitutional convention’ to discuss whether Scottish MPs should continue to vote on English laws. A convention is an unenforceable rule that is respected because people choose to do so. It lasts until those people decide it no longer has any value.

There is still a convention that the law officers will not discuss the advice they have given. But it no longer extends to advice on the legality of military intervention abroad. And if the prime minister can summarise the advice he received in a clear-cut case, he can hardly refuse to disclose it when the legal justification for military intervention is more nuanced.

Changing the convention on the disclosure of legal advice is one thing. But creating a new convention on the use of military action is quite another.

Her Majesty’s armed forces are recruited and deployed under the royal prerogative. In constitutional theory, prerogative powers are exercised on the sovereign’s behalf by her ministers. That has allowed governments to wage war without the need for legislation.

Sending troops into battle must be a difficult decision for any prime minister. Tony Blair may still be haunted by the spectre of Iraq but it is worth recalling that the Commons supported, by 412 votes to 149, his plan to use ‘all means necessary to ensure the disarmament of Iraq’s weapons of mass destruction’.

Blair’s decision in March 2003 to seek parliamentary approval before engaging Saddam Hussein is seen as something of a precedent. In 2011, the government acknowledged that, except in an emergency, MPs should be given an opportunity to debate any proposed use of force. When Cameron wanted to respond to Syria’s use of chemical weapons, he sought MPs’ approval. But when he managed to lose the Commons vote last year, he felt he could no longer launch military action under prerogative powers.

So that probably left us with a new convention binding the prime minister to respect any Commons vote on military intervention. There has never been any suggestion that the Lords should have a say in it.

That convention was endorsed by Cameron when parliament was recalled at the end of last month. He reserved the right to act first and explain his actions to parliament afterwards ‘if there were a critical British national interest at stake’ – which we can take to include cases where military success depends on the element of surprise. But, this time, he made sure he had the Labour party’s support for his plans.

It was one of Cameron’s own backbenchers who pointed out the risks of this convention. Jesse Norman said it was for the government to decide and the Commons to scrutinise. MPs were inevitably far less well informed than ministers. ‘We do not have the same access to officials and advisers; we are not privy to diplomatic traffic or secret intelligence; and we are not briefed by, and may not demand briefings from, our armed forces,’ Norman said.

Once the Commons had voted for military action, Norman added, it could no longer hold the government to account: ministers could simply blame MPs for giving the go-ahead. Far from strengthening parliament, pre-authorisation weakened it.

Norman is right. Cameron should not treat the Commons as a comfort blanket. Of course, a prime minister must command parliamentary support. But MPs are not there to help Cameron sleep better at night. The convention that military adventures require advance approval from MPs should be quietly put to sleep.