A body matching the description of Dr David Kelly - the weapons expert at the centre of the Iraq dossier row - has been found at a beauty spot close to his home in Oxfordshire.
Dr Kelly arriving to give evidence to MPs this week
The government says an independent judicial inquiry will be held into the circumstances of his death if the body is confirmed to be that of the MoD adviser.
The discovery was made at 0920 BST by a member of the police team searching for Dr Kelly in a wooded area at Harrowdown Hill, near Faringdon.
Dr Kelly, 59, had been caught up in a row between the BBC and the government about the use of intelligence reports in the run-up to the war with Iraq.
On Tuesday he told the Foreign Affairs select committee he had spoken to BBC reporter Andrew Gilligan but denied he was the main source for a story about claims that a dossier on Iraq had been "sexed up".
Dr Kelly left his home in Southmoor, Abingdon, Oxfordshire, at about 1500 BST on Thursday and his family reported him missing at 2345 BST the same day.
The body was found lying on the ground, around five miles from Dr Kelly's home, a police spokeswoman said.
Acting superintendent Dave Purnell said formal identification would take place on Saturday and the case was being treated as an "unexplained death".
"We will be awaiting the results of the post mortem and also waiting while
the forensic examination continues at the scene at Harrowdown Hill," he added.
A hearse left the scene shortly before 2000 BST on Friday.
The government announcement of an inquiry if the body is Dr Kelly's came from the prime minister's plane as he flew for a visit to Tokyo.
Mr Blair's spokesman said: "The prime minister is obviously very distressed for the family.
"If it is Dr Kelly's body, the Ministry of Defence will hold an independent
judicial inquiry into the circumstances leading up to his death."
Officials stressed the inquiry would not be the wide-ranging investigation into the run-up to the war urged by opposition MPs.
It will be headed by a law lord - Lord Hutton - but it is expected to take a matter of weeks not months.
Tory leader Iain Duncan Smith said Mr Blair should consider cutting short his trip to the Far East.
Robert Jackson, the Conservative MP in whose constituency Dr Kelly lived, said the "responsibility of the BBC should not go unmentioned" in the case.
"The pressure was significantly increased by the fact the BBC refused to make
it clear he was not the source," he said.
A BBC spokesman said: "We are shocked and saddened to hear what has happened and we extend our deepest sympathies to Dr Kelly's family and friends.
"Whilst Dr Kelly's family await the formal identification, it would not be appropriate for us to make any further statement."
Earlier this week, Dr Kelly denied being the BBC's main source for the story claiming Downing Street had "sexed up" the dossier about Iraqi weapons of mass destruction.
MPs on the Commons foreign affairs committee, which questioned Dr Kelly earlier this week, reacted with shock and disbelief at news of his disappearance.
Huge media attention has been on Dr Kelly since the Ministry of Defence said he had come forward to admit meeting Andrew Gilligan, the BBC correspondent behind the controversial Iraq story.
Mr Gilligan said a source had told him that the dossier on Iraq had been "transformed" by Downing Street.
The BBC correspondent has refused to name his source, but the MoD said Dr Kelly had come forward to say it may have been him.
Supt Purnell said a police family liaison officer is with Dr Kelly's family. He is married to Janice and they have three daughters, Sian, 32,
and twins Rachel and Ellen, 30.
Ann Lewis, a neighbour of Dr Kelly, told BBC News Online she was "devastated".
A police officer in the area where the body was found
She said: "He was a quiet man. He was a man who showed great care and concern for others."
Craig Foster, 36, landlord of the Blue Boar public house in nearby Longworth, said Dr Kelly was "a very well liked gentleman".
A Ministry of Defence spokeswoman said: "We are aware that Dr David Kelly has
gone missing and we are obviously concerned."
The ministry said Dr Kelly had at no point been threatened with suspension or dismissal for speaking to Mr Gilligan.
It was made clear to him that he had broken civil service rules by having unauthorised contact with a journalist, but "that was the end of it", said a spokesman.
Downing Street says "normal personnel procedures" were followed after Dr Kelly volunteered that he might have been the source of Mr Gilligan's report.
It was made clear to Dr Kelly that his name was likely to become public knowledge because he was one of only a small number of people it could have been about, a spokesman said.
After questioning Dr Kelly earlier this week, the Commons foreign affairs select committee said it was "most unlikely" he was the main source for the BBC story.
And they said Dr Kelly, who has worked as a weapons inspector in Iraq, had been "poorly treated" by the government - a charge strongly rejected by the MoD.
Committee chairman Donald Anderson told the BBC his "heart went out" to Dr Kelly's family as the search for the official went on.
Another member of the committee, Tory John Maples said he was "speechless" after hearing of the discovery of a body.
"If it is (Dr Kelly), it is just awful. What can you say? Nothing," he said.
Tory MP Richard Ottaway, another committee member, said: "He is not used to the media glare, he is not used to the intense spotlight he has been put under."