A hospital spokesman said: "Following tests Mr Vaz is being admitted overnight for routine observation."
Mr Vaz, 44, was sitting on the edge of his chair when he suddenly slumped and fell over on to the floor, apparently losing consciousness for about three minutes. He had recovered consciousness before the ambulance arrived.
John Paul Lynch, 22, an outpatient who saw Mr Vaz arrive in an ambulance, said: "He was brought in on a stretcher by about six people. He was taken inside, and he got off the stretcher, but he was quite unstable, weak in the legs. He stood up but he didn't walk, and two people were helping him stand. Then they put him in a wheelchair."
He has been under severe criticism over his handling of his personal finances and for failing to take greater personal interest in the Balkans, one of his areas of responsibility.
Although he has remained outwardly cheerful despite the intense political pressure, observers have noted that he has lost weight.
During the interview, on visas, Jane Faure-Brac, a reporter from the Foreign Office-funded British Satellite News, noted that Mr Vaz looked unwell and agreed at his request to stop filming to allow him to take a break.
After a few minutes he returned but at the end of the interview he collapsed.
Ms Faure-Brac said she loosened Mr Vaz's tie and helped him into a recovery position. "From the moment he came into the room he wasn't himself. He wasn't very focused - there were big pauses between his answers and normally he is very well briefed."
Mr Vaz was criticised earlier this month by the parliamentary commissioner for standards, Elizabeth Filkin, for failing to declare that he had a financial relationship with an Asian lawyer whom he recommended for a personal honour.
A complaint that he failed to declare a donation from a lawyer he recommended for a peerage was upheld by the parliamentary standards committee. He was also criticised for failing to cooperate with the inquiry, but has continued in office with the outward support of Tony Blair. The Tories have made repeated calls for his resignation but Downing Street says he has been cleared of all serious allegations.
He is now, however, facing a further investigation over his wife's involvement with the Hinduja brothers and an alleged failure to declare a relevant interest.
It was also revealed this week that Mr Vaz had approved 49 visa applications against the advice of immigration officers during the 17 months he has been in his Foreign Office job.
On Guardian Unlimited Politics
Ask Aristotle: everything you need to know about Keith Vaz
Related articles
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24.01.2001: The hands-on Hindujas
Profile: Srichand Hinduja
Special report
New Labour in power
Useful links
Keith Vaz home page
Foreign & Commonwealth office Vaz biography
