The former Prime Minister called off the raid by Britain's most elite soldiers in 2013 amid fears it was too dangerous.
Mr Cameron pulled the plug after cabinet discussions because he feared that up to half the elite force of up to 100 troops could have been killed in the secret attack.
But the SAS raids could be revived four years on after the horrifying Sarin attack, which killed 80 men, women and children, it has been claimed.
The head of Britain's special forces has briefed Theresa May's crisis committee Cobra and sources told the Mirror they could still be sent in to do the job after this week's chemical weapons massacre.
MI6 has also asked its operatives in Syria to try to gather samples from the largely opposition-held Idlib province where al-Assad's Sarin attack took place David Cameron's decision to abort the SAS mission came at a time when he wanted Britain to launch air strikes on Syria.
After Assad gassed his own people four years ago, David Cameron lost a Commons vote authorising the use of British military force in Syria.
Ed Miliband, and the leadership of the Labour Party, had looked set to support the PM.
But after realising he did not have the support of the rabid anti-interventionist wing of his party, including the likes of Jeremy Corbyn, he undertook a dramatic volte face.
This decision resulted in a defeat for the government, after 31 Tory MPs joined them, inflicting a humiliating defeat.
The consequences were pivotal. The day after, President Barack Obama put a US military attack on Syria on hold in a dramatic U-turn, despite the gas attack being a flagrant breach of what he had called a ‘red line’. No military action was taken.
Since then, the West’s ability to protect civilians and influence the war has been severely limited.
The following year, Islamic State seized swathes of territory in the country.
Russia moved in to the conflict zone in 2015, exploiting the vacuum left by the US and Europe.
But Donald Trump's order to the US military to fire 59 tomahawk missiles at al-Shayrat military airfield near Homs overnight.
Mrs May has today backed the US action and was told in advance, it was revealed today.
Yesterday Foreign Secretary Boris Johnson urged Donald Trump to push for United Nations backing before resorting to military action against the Syrian regime.
But this morning Theresa May's office said that the missile attack, which has destroyed Assad's airfield, planes and killed at least five people was an 'appropriate response'.
Mrs May was apparently told in advance of the attacks - but was not asked if Britain wanted to join in.
A new Sky News poll this morning suggests that the majority of Britons would now support military action against al-Assad's Syria.
President Trump, speaking from his Mar-a-Lago estate where he is hosting the Chinese President Xi Jinping and his wife Peng as part of a two day summit, said the US had to act after the Syrian dictator launched the 'horrible chemical weapons attack' on innocent civilians.
'Even beautiful babies were cruelly murdered in this very barbaric attack,' he said. 'It was a slow and brutal death for so many.'
Pentagon officials said that the airfield was being used to store chemical weapons and was a base for Syrian air forces - including the aircraft that conducted the chemical weapons attack.
Early reports indicate the strike has severely damaged or destroyed the aircraft and support infrastructure, while at least five people were killed.
A Syrian official claimed the attack killed three soldiers and two civilians, while a Syrian opposition monitor said it killed four soldiers, including a general. Talal Barazi, the governor of Homs province, said seven others were wounded in the early Friday attack.
The Pentagon has released dramatic footage of one of its missile launches from USS Ross, an Arleigh Burke-class destroyer, targeting the Syrian base.
|Abdulhamid Yussef cradles his twins Aya and Ahmad before they are buried in Idlib following a Sarin gas attack CREDIT: ALAA AL-YOUSSEF SOURCED BY JOSIE ENSOR|
Sarin: what is the deadly poison used in the Idlib attacks?
Odourless, tasteless, and absorbed almost instantly on contact in gas or liquid form, Sarin is perhaps the most feared of chemical weapons.
Because it is heavier than air and can saturate material including clothes, it can linger for several hours, especially in low ground.
In Idlib on Tuesday, that was fatal to civilians who took cover from the bombardment underground.
How does it work?
But it doesn’t directly poison its victims – instead, it turns the human nervous system against its owner.
The real killer is something called acetylcholine – a neurotransmitter that our nerves send out to instruct muscles to contract or relax.
It is a process that takes milliseconds, and keeps involuntary organs like heart, lungs, and intestines working day in, day out.
Sarin's deadly trick is to disable the enzyme that normally breaks down the neurotransmitter - leading to rapid build up of surplus messages ordering muscles to act.
Bombarded by intense and repetitive instructions, the victim's muscles try to comply - with undignified, painful, and often fatal consequences.
The immediate symptoms of Sarin exposure - a runny nose, crying, involuntary urination and defecation - are all a result of muscles involved being overloaded with orders to move.
It is not always deadly. But in the absence of antidotes, convulsions, paralysis, and death can follow in minutes.
Most victims, like the children in Idlib, suffocate after losing control of the muscles that control breathing.
What is it for?
Chillingly, it is not simply the weapon of deranged dictators.
During the Cold War, both the Soviet-bloc and Western militaries maintained stocks of deadly chemicals and tactical doctrines about how and when to use them the battlefield.
“Sarin quickly causes casualties over a defined area,” said John Gilbert, Senior Science Fellow at the US-based Center for Arms Control and Non Proliferation.
“And because it dissipates quite quickly, usually within a few hours, you can then send in your own troops to take that area,” he added.
By contrast VX, the nerve agent used to murder Kim Jong Nam, the brother of North Korean leader Kim Jong Un’s brother, in Kuala Lumpur airport in February can hang around in lethal concentrations for days after it has been released, and is viewed by militaries as an “area denial” weapon that can make airports or other areas unusable.
During the Syrian civil war, Bashar al-Assad’s forces have generally used chemical weapons – mostly chlorine gas – to spread confusion in areas where regime troops are on the offensive, said Igor Sutyagin of the Royal United Services Institute.
"The other tactic is like a sniper, lure rescuers to the scene and then hit them. It is a way of concentrating people to the target," he said.
Where is it made?
Sarin is not an easy thing to produce, and if mishandled the process can be extremely dangerous.
But nor is the process impossible to replicate. In 1995, the Aum Shinrikyo suicide cult produced enough in its own lab for a deadly attack on the Tokyo metro.
Syria may have retained some of the stocks it was meant to give up. But it is also conceivable that the regime simply made more, using improvised facilities and ingredients it had not prepared.
Loading it into bombs or rockets to deliver it would be one of the riskiest parts of the operation, but would not be beyond intelligent amateurs with a good grasp of chemistry and who were extremely careful, experts say.
At it's crudest, liquid Sarin could conceivably be loaded in a barrel and thrown out of a helicopter. "You'd just have to consider the crew to be very careful or very expendable. If it leaked it would kill everyone on board," said Mr Gilbert.
Why isn't it banned?
Growing disgust amongst the public, the difficulties of safely storing and maintaining stocks, and the unpredictability of the weapon – a sudden change in wind direction could see the poison blow onto one’s own troops – contributed to the 1993 Chemical Weapons Convention, under which almost every country on the planet has committed to destroy its stocks.
Syria was effectively forced to accede to the convention after the Ghouta Sarin attacks in 2013 – when the United States and Russia compelled Assad to give up his stockpiles.
Officially, the destruction of the Syria’s chemical stockpiles under international supervision was completed in 2014.
It now seems like some of those stocks were overlooked, or deliberately hidden, or replaced by new supplies.