Air strikes in response to an alleged chemical attack risk triggering a “hot war between the US and Russia over the skies of Syria”, Jeremy Corbyn says.
The Labour leader also said Parliament should “always be given a say” on any military action.
US president Donald Trump has warned Russia to “get ready” for missiles being fired at its ally Syria.
Theresa May has said the use of chemical weapons “cannot go unchallenged”.
She said investigations were continuing but that “all the indications” were that the Syrian regime of president Bashar al-Assad, which denies mounting a chemical attack, was responsible.
The UK and its allies were looking at ways to “prevent and deter” the use of chemical weapons, she added.
Speaking during a visit to Plymouth, Mr Corbyn said he had been making the case for Parliament – which is currently in recess for Easter – to be given a say on any military action for “many, many years”.
He said all countries had to get “around the table” to come up with a political solution to the war in Syria, adding: “We cannot risk an escalation any further than it’s gone already.”
Mr Corbyn opposed military intervention in Syria in the 2015 vote when MPs backed action against so-called Islamic State.
Asked what it would take for him to back action this time, he said: “Listen. what happened last weekend was terrible. What we don’t want is bombardment which leads to escalation and a hot war between the US and Russia over the skies of Syria.”
The SNP also called for Parliament to have a say before any UK military action is authorised.
Defence spokesman Stewart McDonald urged the UK and its allies to “clamp down on the development and use of chemical weapons”.
He added: “Air strikes have not prevented these attacks and will not provide the long term solutions needed to end the war.”