Defence Secretary Michael Fallon is expected to tell MPs that Britain must reconsider the question of whether military air strikes against the Islamic State terror group should be extended into Syria.

Mr Fallon said yesterday that it was 'illogical' that UK planes were able to hit extremists in Iraq but not bases across the border. He suggested that any evidence that last week's massacre in Tunisia was planned or ordered in Syria would show that the IS leadership in the country represents a direct threat to the British people.

MPs are not expected to be asked to vote on action in Syria in today's debate on Britain and international security, with some speculation in Westminster that Prime Minister David Cameron would prefer not to seek parliamentary approval until a new Labour leader is in place in September.

The Prime Minister was scarred by defeat in 2013 on his plan to target the forces of Bashar Assad in the wake of the dictator's use of chemical weapons against rebels in Syria, and when the PM obtained Commons approval for the bombing of militant positions last year, he made clear that this was limited to Iraq.

But Mr Fallon said the new parliament elected last month might take a different view, particularly in the light of the murder of 30 UK holidaymakers on a beach in the resort of Sousse by a gunman suspected to have attended an IS training camp in Libya.

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 - 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.

'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 'a lot has happened' since the chemical weapon attack by Assad which sparked the previous proposal for air strikes, '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.'