Keir Starmer has said Tehran can never be allowed to develop a nuclear weapon - as the US bombed three sites in Iran in a massive escalation of the Middle East crisis.

Overnight bunker-busting missiles were dropped from American B-2 stealth bombers, including on Iran's Fordow nuclear site, despite calls for a diplomatic solution.

It is understood the UK was not involved in the action and comes after the Prime Minister repeatedly pushed for a de-escalation in the conflict.

In a statement on Sunday morning, Mr Starmer said: "Iran’s nuclear programme is a grave threat to international security. Iran can never be allowed to develop a nuclear weapon and the US has taken action to alleviate that threat.

“The situation in the Middle East remains volatile and stability in the region is a priority. We call on Iran to return to the negotiating table and reach a diplomatic solution to end this crisis.”

Just days ago Mr Starmer suggested at the G7 summit in Canada the US would not become involved militarily in the conflict between Iran and Israel.

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"There is nothing the President said that suggests he's about to get involved in this conflict, on the contrary, the G7 statement was about de-escalation," he told reporters on Tuesday.

Buisness Secretary also Jonathan Reynolds reiterated the PM's call for a diplomatic solution on Sunday. He told Sky News: "I know people will be waking up this morning, and they'll be worried. They'll want to know what this means.

"And I do want to give them reassurance that whilst the British government, the UK has not been involved in these attacks, we have been making extensive preparations for all eventualities, including how we look after British nationals in the region and how we get them out, and the assets we have in the region to protect British infrastructure, British bases, British personnel if we need to do that."

The Secretary General of the United Nations Antonio Guterres said he was "gravely alarmed" by the US strikes against Iran. He said: "This is a dangerous escalation in a region already on the edge – and a direct threat to international peace and security.

"There is a growing risk that this conflict could rapidly get out of control – with catastrophic consequences for civilians, the region, and the world. I call on Member States to de-escalate and to uphold their obligations under the UN Charter and other rules of international law."

He added: "At this perilous hour, it is critical to avoid a spiral of chaos. There is no military solution. The only path forward is diplomacy. The only hope is peace."

Overnight, Mr Trump posted on Truth Social, saying: "We have completed our very successful attack on the three Nuclear sites in Iran, including Fordow, Natanz, and Isfahan. All planes are now outside of Iran air space. A full payload of BOMBS was dropped on the primary site, Fordow."

Mr Trump also warned the regime in Tehran from retaliating against the US, saying it would be met "with force far greater than what was witnessed tonight".

But Iran said the attack would have "everlasting consequences". Iranian foreign minister Abbas Araghchi said the country "reserves all options to defend its sovereignty, interest and people".

He added: "The United States, a permanent member of the United Nations Security Council, has committed a grave violation of the UN Charter, international law and the NPT by attacking Iran's peaceful nuclear installations.

"The events this morning are outrageous and will have everlasting consequences. Each and every member of the UN must be alarmed over this extremely dangerous, lawless and criminal behavior."