Lord Macdonald, the UK’s former director of public prosecutions, has questioned the legal justification for the drone strike on two British Islamic State fighters said to have been plotting terror attacks in the UK.

He pointed out the events that were reported to have been targeted passed off peacefully before their deaths.

The senior lawyer and Lib Dem peer said it was possible to use self-defence as a legal justification for the drone strikes such as that on Reyaad Khan and Ruhul Amin, which David Cameron told parliament happened on 21 August.

However, Macdonald questioned the timing of the strike if it was meant to protect the UK from terrorist attacks at public events over the summer, such as VE day in May, Armed Forces day in June and VJ day commemorations in mid-August.

“The difficulty is the government is talking about events like VE day being under threat although this drone strike took place several weeks after VE day, which of course passed off without incident. So I don’t think that would pass any test of imminence in law,” he told the BBC.

McDonald said the government needed to be more transparent about the reasons for ordering the strike, including asking parliament’s intelligence and security committee to look at the evidence and ordering the attorney general, Jeremy Wright, to give his point of view to the House of Commons.

“It’s not enough for a minister to come on [BBC Radio 4’s] Today programme and give a set of fairly bland reassurances,” he said. “It’s a departure for us: it’s a new form of warfare, people are concerned about it. There are serious legal considerations. It can be legal, it can be lawful, but we need to be reassured that in this case it was.”

The imminence of the threat was also questioned by Baroness Kennedy, a Labour peer and human rights barrister. She said the government had shifted its ground in arguing that the targets were planning attacks on major public events during the summer, even though the strikes took place in late August.

“Of course you would be entitled to take steps to prevent that imminent threat, but it has to be imminent,” she told ITV’s Good Morning Britain.

The new makeup of the intelligence and security committee has just been announced by the prime minister, and includes former attorney general Dominic Grieve, former adviser Keith Simpson, former minister Sir Alan Duncan, SNP Westminster leader Angus Robertson, and Labour MPs George Howarth, Fiona MacTaggart and Gisela Stuart.

One of their first tasks is likely to be scrutiny of the drone strikes. It emerged on Tuesday that unmanned RAF aerial drones armed with Hellfire missiles have been patrolling the skies over Syria for months, seeking to target British jihadis on a “kill list” drawn up by senior ministers on the UK national security council shortly after the election.

Labour leadership contender Jeremy Corbyn led a cross-party group of MPs who raised doubts about the change in strategy. “There has to be a legal basis for what’s going on. This is war without parliamentary approval. And, in fact, parliament specifically said no to this war in September 2013,” he said.

Senior Liberal Democrats suggested the RAF drone strike went beyond anything that would have been approved when former party leader Nick Clegg sat on the NSC.

