David Kelly, 59, went missing from his home near Abingdon, west of London, at around 3:00 pm (1400 GMT) on Thursday, said local police who were alerted after he had failed to return home by late evening.

"He told his wife that he was going for a walk and he never came back," said police spokeswoman Kate Smith, adding that a major seach operation had been launched.

Kelly faced a grilling on Tuesday by legislators in the House of Commons' foreign affairs committee about what he told BBC reporter Andrew Gilligan, who wrote the original report claiming the government had altered the weapons dossier.

He denied being the main source for Gilligan's report.

Opposition Conservative legislator Richard Ottaway, who was on the committee, said Kelly had alluded on Tuesday to the level of pressure he was under.

"He was asked to provide some evidence and he replied that he would do so but he could not get into his house because of the media pressure," Ottaway said.

"Let's hope that nothing sinister has happened on this occasion and that he has simply taken a step to get away from the pressure," Ottaway said.

Police have given out a description of Kelly and said that he would have been distinctive as there was bad weather and it was unlikely that he had a coat with him.

In his BBC radio report in May, Gilligan claimed that Alastair Campbell, the government's director of communications and key Blair aide, had ordered that the headline-making claim that Iraq under Saddam Hussein could deploy chemical or biological weapons in as little as 45 minutes, be inserted into the government dossier released last September.

The report sparked a furious row with the government, prompting the official parliamentary inquiry into the intelligence presented by Downing Street as a justification for joining the US-led war on Iraq in March.

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