Prime Minister Sir Keir Starmer has urged further negotiations rather than an escalation of the Israel-Iran conflict.
His latest call comes as President Donald Trump considers whether to join Israeli strikes, with the BBC's US partner CBS reporting he has approved attack plans but has not made a final decision.
Conservative shadow foreign secretary Dame Priti Patel has called on the UK government to back the US if it attacks Iran, as has former Conservative foreign secretary Jeremy Hunt.
However, Britain's top legal adviser Lord Hermer has reportedly raised concerns about UK involvement in some of a range of scenarios.
Responding to reports advice included that UK military action should be limited to defensive support, a senior government source told the BBC: "It is not as simple as that."
Dame Priti has argued the UK cannot "hide behind legal advice at a time of crisis".
But Liberal Democrat leader Sir Ed Davey called for any legal advice to be published, adding: "The last thing we need is for the UK to be dragged into another illegal war in the Middle East by the US."
Asked about the potential for UK involvement with any US strike, Sir Keir said the "nuclear issue needs to be dealt with but it is better dealt with by way of negotiation than by way of conflict".
Pressed on any legal advice he may have received from Lord Hermer, the prime minister said "the Attorney's advice is never disclosed by any government" but that "the principle, the driving intent is de-escalation".
Sir Keir added: "all of us are very concerned about the nuclear programme Iran is developing.
"We also recognise Israel's right to self-defence," he said, adding: "We need to de-escalate. The risk of escalation in the region is obvious."
Foreign Secretary David Lammy is in Washington to meet his US counterpart Marco Rubio, initially scheduled for Monday, when Lammy chaired a meeting with Iran and EU representatives about the escalation of Iran's nuclear programme.
The pair will be seeking to smooth over a disagreement last week, when Rubio condemned the UK's decision to sanction two far-right Israeli cabinet ministers, alongside France, Germany, Canada and Australia.
Lammy will then travel to Geneva for talks with the EU's top diplomat, Kaja Kallas, his counterparts from Germany and France, and Iran's foreign minister Abbas Araghchi, who has insisted his country was committed to diplomacy.
Source: BBC
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