The former prime minister said Theresa May should be prepared to support potential US strikes because “non-intervention is also a policy with consequences”.
Claiming that UK action against the Syrian regime is “highly likely”, Mr Blair said the government would not necessarily need to seek parliament’s approval for air strikes against the Assad regime.
Talk of possible UK military action follows reports that chemical weapons were used in the formerly rebel-held town of Douma.
Medical agencies say dozens of people were killed in the attack, which has been widely blamed on the Assad regime.
“Nothing is off the table”, he said. “It will be met, and it will be met forcefully.”
Mr Blair said the UK government should “support US action”. He told BBC Radio 4’s Today programme: “If the US are taking action, we should be prepared to be alongside them.
“Last year they took military action and that was supposed to deter these attacks. It obviously had some impact, but now we have another one of those attacks.”
He added: “It’s important to realise that if we allow it to go unchecked and unanswered, then obviously the Assad regime and their outsider backers in Russia and Iran will feel emboldened to do more, so I think it is important that we react.
“If the Americans are prepared to act and are going to act fast, I think ourselves – probably the French government will be in the same position – should be supportive, because it’s important that when chemical weapons are used in this way and the international community has taken a firm position against it, then you have to enforce it.”
The US hit Syrian air bases with dozens of missiles last April after intelligence suggested the Assad regime was responsible for using chemical weapons in the town of Khan Shaykhun in Idlib province.
The latest attack also resulted in a strike against a Syrian base, which Russia and Syria claimed was carried out by Israel.
Mr Blair warned that failure to take further action against Assad would have “consequences”.
“It’s extremely difficult but the problem is non-intervention is also a policy with consequences,” he said.
“Iraq shows you intervention is extremely difficult but non-intervention, as we see from Syria, is also extremely difficult.”
He added: “If you say, ‘Well suppose we do nothing’, then you really are giving carte blanche to the regime to do whatever is necessary to retake opposition areas by force.”
The Syrian government is winning the civil war in the country “because their outside backers are prepared to intervene and to commit, and frankly we have not been able now,” he said.
The attack in Douma took place on Saturday, as Syrian forces attempt to regain territory following the collapse of a ceasefire with the Army of Islam rebel group.
The Assad regime denies that chemical weapons were used, but videos shared on social media showed numerous dead children with foam on their mouths.
Reports suggest medics treated more than 500 people, most of them women and children, for symptoms including difficulty breathing, foaming at the mouth and burning sensations in the eyes.
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