MPs could be asked to authorise airstrikes against Islamic State (IS) in Syria in the wake of the Tunisia massacre, the defence secretary has suggested.
Michael Fallon said it was "illogical" that UK planes were able to hit extremists in Iraq but not bases across the border.
The Commons gave the go-ahead to bombing of militant positions in Iraq last year, but MPs were not asked to approve wider action.
Speaking on BBC Radio 4's World at One, Mr Fallon said work was under way to see if the attack in Tunisia last week had been directed from Syria.
Asked whether he thought British forces should have the scope to strike IS – also known as Isil - targets on that side of the border, he said: "There is an illogicality about not being able to do it.
"There were reservations in the last parliament about doing anything in Syria that would prop up the Assad regime, which of course partly caused this problem in the first place ...
"It is a new parliament and I think new Members of Parliament will want to think very carefully about how we best deal with Isil, and the illogicality of Isil not respecting the borderlines – they don't differentiate between Syria and Iraq, they are establishing this evil caliphate across both countries.
"There is no legal bar to us operating in Syria but we don't have the parliamentary approval for it.
"We don't need it at the moment because we are playing our part in the campaign, and indeed what we do in Iraq actually frees up the US to attack in Syria."
He added: "We have made it clear we would have to go back to parliament, yes, and ask for parliamentary authority because we don't have that at the moment.
"The exception to that, as the Prime Minister has always made clear is where we think there is an imminent threat, a very direct to British lives or for example to British hostages.
"Then we reserve the right to take action without prior parliamentary approval and then coming to account for it afterwards ...
"Isil has to be defeated in both countries and all its evil in Iraq is all being directed by its headquarters in Syria."
Mr Fallon said efforts were under way to uncover links to the beach attack in Sousse, in which 30 Britons are thought to have died.
"If we can link it back, if it does link directly back to Isil in Syria, then we will have to reflect with the rest of the coalition how best we deal with that," he said.
The Prime Minister's official spokeswoman said: "The Prime Minister has been clear on the need for us to be crushing Isil in both Iraq and Syria.
"Clearly, Isil is seeking to find areas from which it can operate, from where it can seek to threaten people here in Britain, and as part of what the PM was talking about in terms of having a 'full spectrum' response, that clearly means not just focusing on one area where they are, but looking at a whole range of areas and how Isil are operating."
The spokeswoman said that "a lot has happened" since the chemical weapon attack by Syrian president Bashar Assad which sparked the previous proposal for airstrikes, "most recently 22 and likely more British citizens have been killed in a terrorist attack that the investigation so far suggests there are links to Isil".
She added: "Therefore the Prime Minister is clear that we absolutely should be thinking about are we doing enough, and in the right areas, to tackle Isil."