The Department of Health has apologised for an apparent security lapse which allowed the personal details of junior doctors to be accessed online.
Junior doctors have been protesting about MTAS
Channel 4 News reported that a breach on the NHS Medical Training Application Service website allowed public access for at least eight hours.
The department said the details had only been available briefly, and only to people making employment checks.
Shadow health secretary Andrew Lansley said it was shocking and unacceptable.
On Wednesday, Channel 4 News reported that a doctor had alerted them to a security breach allowing confidential details to be accessed.
Phone numbers, addresses, previous convictions and sexual orientation were among details available since at least 0900 BST, it reported.
The Department of Health was alerted at 1635 BST and the breach closed at 1705 BST.
In a statement, it apologised and said it was "a very serious matter and is under investigation".
It said the web address was only made available to a "strictly limited number of people" and only for a short period of time.
Health Minister Lord Hunt told BBC Radio 4's Today programme the information appeared to have been deliberately leaked.
He added: "What I can assure you is we've got an urgent investigation taking place, because we clearly have to find out how that information was accessed improperly, whether any criminal offence has taken place and what the lessons are to be learned.
"Yes, it is serious, and we're determined to get to the bottom of it."
On Thursday, NHS IT advisor Dr Gillian Braunold appeared before MPs and told them some of the software designed to prevent hacking was to be independently evaluated.
The MTAS website, used to select candidates for specialist training, was already at the centre of a row between doctors and the government.
Doctors say it is failing to select the best candidates and thousands have been left without any interview at all.
Junior doctor Alex Liakos is among those whose details were on the site. He told the BBC the latest problem was "the final nail in the coffin".
"This proves to all doctors up and down the country that the government is completely incompetent in allocating us to our training posts and should be nowhere near it."
He backed calls to prosecute the Department of Health under the Data Protection Act.
Mr Lansley said the breach should never have happened.
"It's bad enough that junior doctors have their future career in the balance, but to have their private details accessible on a website for all to see is deeply regrettable."