A statement said Mr Austin was transported to Walter Reed National Military Medical Center, in Maryland, at around 2.20pm.

The 70-year-old "is retaining the functions and duties of his office", the Pentagon added, with the deputy secretary Kathleen Hicks on standby if required.

Mr Austin is just below the president in the military chain of command and must be on call to immediately respond to any crisis.

The White House and Congress have been notified, along with the deputy secretary of defence and the chairman of the Joint Chiefs of Staff, the statement said.

It added that Mr Austin travelled to hospital with the unclassified and classified communications systems needed to perform his job.

The defence secretary was diagnosed for prostate cancer in early December, and had surgery on 22 December, but did not tell the President Joe Biden and other senior figures until days after complications forced him into intensive care on 1 January.

He admitted he "did not handle [the situation] right" and said he should have told the president.

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His slow disclosure of his condition prompted an internal Pentagon review and an inspector general review into his department's notification processes.

Mr Austin was taken back to hospital by ambulance with severe pain 10 days after his surgery.

President Biden previously said it was poor judgement from Mr Austin not to tell him he was in intensive care.

However, he said he still had confidence in him.

Mr Austin was scheduled to depart for Brussels on 13 February to hold a meeting of the Ukraine contact group, which he established in 2022 to coordinate military support for Kyiv after Russia's invasion.

After that engagement, he was scheduled to attend a regular meeting of NATO defence ministers.

It is not clear if the hospital admission would change those plans.