There is “a very high degree of probability” that Tehran was behind the devastating attack that shut down half of Saudi oil production and has raised fears of war, the prime minister said.
Images of the site show “remnants of Iranian-made cruise missiles”, a UK official added, making the claim of responsibility by Houthi rebels in Yemen “implausible”.
Mr Johnson said he had not yet decided on a response, but pointed to an American proposal to “do more to defend Saudi”.
“We will be following that very closely and clearly, if we are asked, either by the Saudis or by the Americans, to have a role, then we will consider in what way we could be useful,” he told reporters.
Until now, the UK has declined to echo Mr Trump’s blaming of Iran for the incident on 14 September, which prompted him to warn the US stood “locked and loaded” to take military action.
But, speaking en route to the UN in New York, Mr Johnson said: “The UK is attributing responsibility, with a very high degree of probability, to Iran for the Aramco attacks.
“We think it very likely indeed that Iran was indeed responsible using both drones and cruise missiles.
“The difficulty is how do we organise a global response, what is the way forward, and we will be working with our American friends and our European friends to construct a response.”
He insisted the aim of that response would be to “de-escalate tensions in the Gulf region” – despite the possible offer of military help for Riyadh.
Despite his initial bellicose response, the US president has since drawn ridicule for holding back on punishing Iran for the attacks, dismaying the Saudis.
Instead, Mr Trump has made clear that Riyadh would have to take the lead military role and even pay for whatever action America took on its behalf.
Mr Johnson said his aim was to “bring the world together in response to what happened in Saudi Arabia”.
Asked if action could include tighter sanctions on Iran, he replied: “There is certainly a case for responding together and that is what we are going to do.”
Mr Johnson also made clear he would challenge Hassan Rouhani, the Iranian leader, when the pair hold talks in New York – as well as protest about the continued imprisonment of Nazanin Zaghari-Ratcliffe.
Iran must “release not just Nazanin Zaghari-Ratcliffe but others who are, in our view, being illegally and unfairly held in Tehran”, the prime minister said.