The Labour leader's condemnation came as the SNP's Nicola Sturgeon warned that the airstrikes could make the seven-year civil war in Syria worse, and accused Ms May of "complying with Presidential wishes".
Parliament should have been consulted and voted on the matter.
Mrs May on Thursday won backing from her senior ministers to take unspecified action with the U.S. and France to deter further use of chemical weapons by Syria.
But rival politicians and some Conservative colleagues have called for a parliamentary vote before any British involvement.
BBC political editor Laura Kuenssberg said it was understood the United Kingdom was planning a genuinely joint effort with the USA and France, rather than playing a supporting role.
U.K. Labour leader Jeremy Corbyn warned Friday that British military intervention in Syria risks escalating an "already devastating conflict" and accused Theresa May's government of "waiting for instructions" from the US president on how to respond to Saturday's alleged chemical weapons attack.
In a post made on Twitter, Johnson said: "Welcome the news of United Kingdom military strikes against major chemical weapons facilities in Syria alongside our USA and French allies".
Corbyn is right, bombs will not save lives.
But Mr Corbyn said military action was unlikely to solve the situation.
Mr Cable said his party had been willing to back a "properly planned and justified" intervention in Syria, but he warned today that the decision to move ahead without Parliament "fatally undermines the integrity of this mission".
"The reason they're not doing it is they are frightened they'll lose the vote".
Pushed on the point by Today, Ms Abbott said: "It's clear, that at this point, Russian Federation - its role in Syria... its role in the poison gas attack in Salisbury - is a greater threat to world peace than the United States".
The action following an alleged chemical-weapons attack on the rebel-held town of Doumaon April 7 was condemned by several British opposition parties, who wanted parliament recalled.
Mrs May and Mr Trump had also "agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime".
- Demand we stop this war.
Liberal Democrat leader Sir Vince Cable demanded Parliament be recalled to vote on the crisis, telling the BBC: 'The position is a very risky one because of Russian involvement - also because we have an erratic president of the United States'.