Foreign Secretary David Lammy will hold talks with the Iranian Foreign Minister on Friday as part of efforts to end the Israel-Iran conflict.
Mr Lammy will meet Abbas Araghchi in Geneva alongside his counterparts from France, Germany and the EU as he seeks an end to the crisis before US President Donald Trump decides on whether to take military action against Tehran.
Explained: What is going on between Israel and Iran?
A statement from Mr Trump on Thursday said there was still "a substantial chance of negotiations" and said he would make a decision on deploying US forces "within the next two weeks".
Mr Trump had previously said he "may" join Israeli strikes against Iran and its nuclear programme, but added: "I may not do it. I mean, nobody knows what I'm going to do."
Friday's meeting with the so-called E3 countries follows Mr Lammy's visit to Washington, where he met US secretary of state Marco Rubio in the White House on Thursday evening to discuss "how a deal could avoid a deepening conflict".
The Foreign Secretary said: "The situation in the Middle East remains perilous. We are determined that Iran must never have a nuclear weapon."
Adding that a "window now exists within the next two weeks to achieve a diplomatic solution", Mr Lammy said: "Now is the time to put a stop to the grave scenes in the Middle East and prevent a regional escalation that would benefit no one."
The conflict continued on Friday with the Israeli military saying it has attacked dozens of military sites in Iran overnight, while Iran also launched missiles towards Israel.
The Israeli strikes included an attack on the Organization of Defensive Innovation and Research (SPND), which it said is involved in Iran's nuclear weapons development.
Israeli airstrikes reached into the city of Rasht on the Caspian Sea early on Friday, Iranian media reported.
Five people have been injured in Iran strikes on Beersheba, southern Israel, according to emergency services, with fires burning close to Microsoft's offices.
Since the conflict erupted last week, at least 657 people, including 263 civilians, have been killed in Iran and more than 2,000 wounded, according to a Washington-based Iranian human rights group.
Meanwhile, at least 24 people in Israel have been killed and hundreds wounded.
Earlier on Thursday, Prime Minister Sir Keir Starmer had urged the US to step back from military action, saying there was a "real risk of escalation".
It remains unclear whether the UK would join any military action, although there has been speculation that US involvement could require using the British-controlled base on Diego Garcia in the Chagos Islands.
The B-2 stealth bombers based there are capable of carrying specialised "bunker buster" bombs which could be used against Iran's underground nuclear facility at Fordo.
Attorney General Lord Hermer is reported to have raised legal concerns about any British involvement in the conflict beyond defending its allies, which could limit the extent of any support for the US if Mr Trump decides to act militarily.
Meanwhile, two Labour backbenchers pushed for a "fresh, tough approach" to Tehran.
Jon Pearce and Mike Tapp, chairman and vice-chairman respectively of Labour Friends of Israel, said the UK urgently needed "a multifaceted diplomatic, economic and national security plan to guard against the Iranian threat and force the regime to change course".
Writing in The Daily Telegraph, the pair called for tighter sanctions on Iran, the proscription of the country's Revolutionary Guard Corps and a "comprehensive diplomatic solution" that "eliminates once and for all" Iran's nuclear threat.
