PA MediaOne of the UK's two aircraft carriers has been placed on advanced readiness to sail from Portsmouth, the BBC understands.
The crew of HMS Prince of Wales have been told they must be ready to leave in five days, defence sources said.
This may raise speculation the carrier could be deployed to the Mediterranean to help defend British interests threatened during the conflict in the Middle East.
Before this, the ship's "notice to sail" was 14 days.
Meanwhile, more US B-1 Lancer bombers have arrived at RAF Fairford.
The government has been accused of not acting fast enough to protect Cyprus from enemy drones and missiles.
Prime Minister Sir Keir Starmer denied the UK was not prepared for the conflict, saying Britain had begun pre-deploying assets to the region earlier this year - particularly to Qatar and Cyprus - alongside the US and other allies.
Defence sources said 400 additional UK personnel had been sent over the past weeks to support air defence activities across the UK's bases in Cyprus.
In other developments:
A Type 45 destroyer, HMS Dragon, has been deployed to the region but will not be ready to leave Portsmouth until next week.
HMS Prince of Wales is in Portsmouth receiving routine maintenance ahead of a planned deployment to the North Atlantic and Arctic later this year.
The carrier strike group was due to take part in Operation Firecrest alongside US, Canadian and European allies to deter Russian aggression in the High North.
A defence source told the BBC the Prince of Wales had had "her readiness increased to five days' notice to sail".
The Ministry of Defence (MoD) confirmed the change in status for the carrier.
A ministry spokesperson said: "We have been bolstering our UK military presence in the Middle East since January, and we have already deployed capabilities to protect British people and our allies in the region, including Typhoons, F-35 jets, air defence systems and an extra 400 personnel into Cyprus.
"Since the strikes began, we've had British jets in the sky shooting down drones and have sent additional assets to the region to further reinforce our air defences, including more Typhoons and Wildcat helicopters with drone busting missiles.
"HMS Prince of Wales has always been on very high readiness and we are increasing the preparedness of the carrier, reducing the time it would take to set sail for any deployment."
PA MediaThe MoD also confirmed the US had started using British bases "for specific defensive operations".
The first B-1 arrived at RAF Fairford in Gloucestershire on Friday evening, followed by three more of the jets and a C-5 airlifter on Saturday.
Dozens of anti-war protesters gathered outside the base, carrying flags and placards. A smaller group of people also attended in support of the UK military.
In London, thousands of protesters called for an end to attacks on Iran, which they said were illegal, unprovoked and unjustified. Marching towards the US Embassy, they held placards saying "Hands off Iran" and "Stop Trump's Wars".
In a separate protest headed from Downing Street to the Iranian Embassy, other demonstrators called for a free and democratic Iran.
While much of the anger on the anti-war march was aimed as the US president, protesters were clear that the UK should not blindly follow the US into a war in the Middle East.
Sir Keir has come under mounting criticism for his approach to the crisis. Last week, he defended his decision not to permit the US to use British bases in the opening assault against Tehran, arguing that the government must keep a "cool head". But on Saturday, Conservative leader Kemi Badenoch renewed her attacks on the prime minister's approach, saying he was "too scared to make foreign interventions."
EPA
