Prime Minister Boris Johnson has said Iran’s admission that it shot down a passenger plane by mistake is an “important first step”.
In a statement, he said there should now be an independent international investigation and the repatriation of those who died in the crash.
Mr Johnson called the downing of Ukraine International Airlines flight PS752 a “tragic accident”.
Iran has admitted it was shot down due to “human error”.
Tehran initially denied responsibility for Wednesday’s crash – which killed 176 people, including four Britons.
But President Hassan Rouhani earlier admitted Iranian military had “unintentionally” shot down the passenger plane after it was mistaken for a cruise missile when it turned towards a sensitive military site.
President Rouhani said the missile strike was an “unforgivable mistake”.
The crash came just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.
Mr Johnson said: “This will be an incredibly difficult time for all those families who lost loved ones in such tragic circumstances.
“We will do everything we can to support the families of the four British victims and ensure they get the answers and closure they deserve.”
He said the investigation into the crash should be “comprehensive, transparent and independent”, adding that he would work with other affected nations, including Canada and Ukraine.
Mr Johnson said it was “vital that all leaders now pursue a diplomatic way forward” in the conflict in the region.
“This tragic accident only reinforces the importance of de-escalating tensions in the region. We can all see very clearly that further conflict will only lead to more loss and tragedy,” he said.