MPs should consider allowing Britain to bomb Islamic State targets in Syria, the defence secretary is to say.
The RAF has been carrying out strikes in Iraq since September but Michael Fallon will say Parliament should look at the case for missions in Syria too.
Mr Fallon will say there is no legal barrier but assure MPs no action will be taken without a Commons vote.
He will suggest terrorist attacks, such as Friday’s tourist murders in Tunisia, may have been planned by IS in Syria.
Twenty-nine of the 38 tourists killed on the beach in Sousse on 26 June have been confirmed as British. Student Seifeddine Rezgui, 23, said to have had links to IS, was shot dead by police after carrying out the attack.
Prime Minister David Cameron later said IS posed “an existential threat” to the West, and its members in Iraq and Syria were plotting “terrible attacks” on British soil.
Mr Cameron was defeated in the Commons in 2013 when Tory rebels joined forces with Labour to oppose air strikes on Syrian government targets designed to deter the use of chemical weapons.
Parliament approved UK bombing of militant positions in Iraq last year. However, MPs were not asked at the time to authorise strikes across the border in Syria.
BBC political editor Nick Robinson said ministers would not risk losing another vote on Syria unless they knew they had the support of Labour.
He added that he understood there had been contact between the two parties to discuss the possibility of another vote on Syrian military action in the future though nothing was expected imminently.
On Wednesday, Mr Fallon told the BBC there was an “illogicality” of British forces observing the Iraq-Syria border when IS “don’t differentiate” between the two countries and moves freely between them.
Speaking on BBC Radio 4’s The World At One, he added: “There’s no legal bar to us operating in Syria but we don’t have the parliamentary approval for it.”
The Iraqi government requested allied military support in its fight against Islamist militants but the Syrian government of Bashar al-Assad, whose country is in the throes of a four-year civil war, has not made a similar request.
On Wednesday, the bodies of eight of the British tourists shot dead in the Tunisia beach attack arrived back in the UK at RAF Brize Norton in Oxfordshire.
The coffins of relatives Adrian Evans, Charles (known as Patrick) Evans and Joel Richards, as well as Carly Lovett, Stephen Mellor, John Stollery, Denis Thwaites and Elaine Thwaites, were flown from Tunis on an RAF C17 and taken off the plane one by one.
The repatriation of all the bodies is likely to take several days.
Downing Street has ruled out an inquiry into the attack, but said Scotland Yard would assist the Tunisian investigation.
A minute’s silence will be observed across the country at noon on Friday in memory of the victims, and matches at Wimbledon will start later so players, spectators and staff can take part.
Tunisia beach attack: The victims
Most of the Britons killed have now been named. Here’s what we know about those who lost their lives, as well as those still unaccounted for and the injured.
Survivors have also been speaking about their ordeal.