Trump and May spoke after May's senior ministers gave her their backing to take unspecified action with the United States and France to tackle the use of chemical weapons by the government of Syria's President Bashar Assad.
The shadow of the 2003 invasion of Iraq still lingers in the corridors of Britain's parliament, when MPs backed then-prime minister Tony Blair in joining United States military action. "Cabinet agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime", the Downing Street said in a statement.
May added: "The Syrian Regime has a history of using chemical weapons against its own people in the most cruel and abhorrent way".
"I have done so because I judge this action to be in Britain's national interest". He described the main aim as establishing "a strong deterrent" against chemical weapons use.
"So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime", she said.
The Russians and many western commentators insist that Assad does not need to use chemical weapons as he clearly is set to finally win the seven year civil war that has claimed half a million lives.
May's office published a summary of the legal advice about the Syria action on Saturday, which said that global law allowed Britain to act "on an exceptional basis" to "alleviate overwhelming humanitarian suffering".
She added: "We agreed that is was both right and legal to take military action together with our closest allies".
It said initial indications were that the precision weapons and meticulous target planning had "resulted in a successful attack".
Or it was the precursor to wider military action.
US President Donald Trump and Britain's Prime Minister Theresa May agree on the need to find an worldwide response to deter the use of chemical weapons in Syria, the British prime minister's office said on Thursday.
Images from the satellite imagery company DigitalGlobe now appear to show at least two of these three sites - the Barzeh Research and Development Center and the Him Shinshar chemical weapons storage complex - before and after the strikes.
When asked if Syria's Assad could remain leader as long as he refrained from further use of chemical weapons, May said: "This was about, as I have said and you have recognized, this was specifically about the use of chemical weapons".
"Most worrying, is that she has acted at the behest of presidential tweets and sidelined parliament", he said.
Russian Federation has blocked moves for a United Nations investigation into the Douma attack and even made the "grotesque and absurd claim that it was staged by Britain", May said.
British MPs voted against taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
Jeremy Corbyn and other opposition leaders have called for Parliament to be given a vote before any decision is taken.
Ms May recalled ministers from their Easter holiday to debate action over what she has cast as a barbaric poison gas attack by Syrian government forces on civilians in the formerly rebel-controlled town of Douma, just east of the capital Damascus.
Britain has been launching air strikes in Syria from its military base in Cyprus, but only against targets linked to the Islamic State militant group.