The former head of the Army has said Britain must work with the Syrian President Bashar al-Assad to combat the Islamic State (Isis).
Lord Dannatt, the former Chief of General Staff, called on the West to reconsider its relationship with the leader, who was internationally condemned for a crackdown on civil liberties during the 2011 Arab Spring.
“The Syrian dimension has got to be addressed. You cannot deal with half a problem,” he told BBC Radio 4’s Today programme.
“The old saying my 'enemy's enemy is my friend' has begun to have some resonance with our relationship with Iran.
"I think it's going to have to have some resonance with our relationship with Assad."
Britain joined the EU, US and majority of the Arab League calling for his resignation and his administration been implicated in war crimes and atrocities including a chemical weapons attack that killed hundreds of civilians near Damascus in August 2013.
David Cameron narrowly lost a crucial Commons vote on military intervention in Syria in the wake of the atrocity last year and had called for a “tough response” against al-Assad’s "illegitimate" regime.
But now the President’s army is one of the last bastions fighting against the advance of Isis in Syria, where it gained a foothold before starting its bloody sweep through northern Iraq.
The Syrian Government had been accused of turning a blind eye to the jihadists to let them fight the mainstream rebel groups it also opposes, but stepped up a campaign of air strikes against Isis headquarters in recent weeks.
Lord Dannatt said a conversation had to be held with al-Assad, whether “above the counter or below the counter”.
"If there are going to be any question of air strikes over Syria airspace it's got to be with the Assad regime's approval,” he added.
“Who actually understood that country, Syria, best - was it us, was it other people, or was it Assad himself?
"It's clearly turned out over the last two or three years to be a very diverse, very complicated country.”
Joining calls for Parliament to be recalled to discuss the escalating crisis in Iraq and Syria, Lord Dannatt said he believed more UK special forces may be needed on the ground in Iraq to train Kurdish troops.
He also suggested the “time will come” when the Government decides that British planes should carry out air strikes, rather than leaving it to the US.
The Foreign Secretary, Philip Hammond, was questioned on the prospect of co-operation with al-Assad on the Today programme on Wednesday, in the wake of James Foley’s brutal murder.
“One of the observations I made on one of my earlier visits to the Middle East is that it is a part of the world where the old adage that ‘my enemy’s enemy is my friend’ simply does not apply,” he said.
“The politics of the Middle East are very complicated. In some cases we will find ourselves coincidentally fighting people who are also being fought by people who we would not share any space with at all and President al-Assad is one of those.”
Mr Hammond did not rule out Britain fighting organisations that are also fighting the Syrian regime but distanced the Government from the possibility of direct co-operation.
“I don’t envisage us having any kind of relationship with the Assad regime but that it is not to say that we may not find ourselves fighting common enemies,” he added.