Foreign Secretary Yvette Cooper condemned Iran’s missile strikes on Diego Garcia, while downplaying wider UK action.
Iran has developed long-range missiles capable of reaching major European cities, Israel has warned.
Iran targeted Diego Garcia in the Chagos Islands before the Government gave the US permission to strike missile sites targeting the Strait of Hormuz on Friday, but it is not clear exactly when the two intermediate-range ballistic missiles were fired.
The Israeli Defence Forces (IDF) said it was the first time Tehran had launched a long-range missile since the start of the war, and showed it is now capable of reaching cities like London, Paris or Berlin.
“We have been saying it: The Iranian terrorist regime poses a global threat,” the IDF said in a statement.
The Israeli military claimed the missiles could reach a distance of around 4,000km, posing a danger to dozens of countries in Europe, Asia and Africa.
Foreign Secretary Yvette Cooper condemned Iran’s missile strikes on the joint US-UK base on Saturday while stressing that the UK will not be drawn into a wider conflict.
She said ministers support defensive action only and want to see a swift resolution to the war.
One of the missiles was shot down by a US warship while the other failed in flight, the Wall Street Journal reported.
Conservative leader Kemi Badenoch urged the Prime Minister to “come clean” about the details of the latest attack on British troops and explain why the public was not informed.
“Keir Starmer has dithered and delayed on the Iran conflict from the outset,” she said.
“Now we find out, from the media and not the Prime Minister, that the British base on Diego Garcia has been the target of Iranian missile attacks.
“The Prime Minister needs to immediately come clean about the details of this latest attack on British troops and explain why the public weren’t informed sooner.”
The Ministry of Defence (MoD) has so far declined to comment on when the attack was launched.
Sir Keir spoke to Cypriot president Nikos Christodoulides on Saturday morning, reiterating that RAF Akrotiri will not be used by the US to target Iranian missile sites.
He also spoke to the Crown Prince of Bahrain, Salman bin Hamad Al Khalifa, to discuss the war.
Diego Garcia is around 3,800km (2,360 miles) away from Iran and home to an airbase capable of accommodating long-range US bombers.
The UK has agreed to cede sovereignty of the islands to Mauritius and lease back the base.
Diego Garcia is strategically valuable to the US and has been used as a launchpad for operations in the Middle East for years.
It has a large airfield, major fuel storage facilities, radar installations and a deep-water port and is home to about 2,500 mostly American personnel.
Iran has escalated attacks on its Gulf neighbours in recent days while keeping a stranglehold on shipping in the Strait of Hormuz.
The Prime Minister will hold a Cobra meeting next week to discuss plans to help households with cost of living issues caused by the war, it is understood.
