Intelligence sources were briefing journalists in a "widespread fashion" about their concerns over the government's presentation of the case for war against Iraq, BBC defence correspondent Andrew Gilligan has told MPs.
The committee is examining the case for war in Iraq
Mr Gilligan was giving evidence to the Commons foreign affairs committee, which is investigating the case made by the government for war in Iraq.
The journalist's evidence came just before it emerged that MPs had again written to Tony Blair's director of communications, Alastair Campbell, again asking him to appear before the committee.
But the request appeared to draw a blank with Downing Street saying: "As we have made clear in the past, the prime minister
believes there is a convention that officials from No 10 do not appear before
select committees to talk about their work."
The BBC correspondent reported last month that a senior British official had told him that a dossier on Iraq's weapons of mass destruction programme had been "sexed up" at the request of Downing Street.
The source said he believed a claim in the dossier - that Iraq possessed chemical and biological weapons which could be deployed 45 minutes after an order to do so - was unreliable.
And Mr Gilligan said his source told him that Tony Blair's director of communications, Alastair Campbell, was responsible for transforming the way the intelligence services' information had been presented in the dossier.
Mr Gilligan's report on BBC Radio 4's Today programme sparked the ongoing row about the government's presentation of the case for war.
The BBC correspondent told MPs he did not know what the motives of intelligence sources were in highlighting their concerns.
He said other BBC reporters as well as a host of newspaper journalists were also told by intelligence sources of concern over the "tone and tenor of the dossier".
Asked by MPs if he felt such sources were seeking to undermine the government, he said: "I have no opinion and I have no evidence to judge whether the intelligence agencies were seeking to undermine government policy."
Mr Gilligan said his Today programme report on the 45 minutes claim was based on a conversation of between 90 minutes and two hours with a British official who was a "long-standing contact".
He described the official as "quite closely connected with the question of Iraq's weapons of mass destruction"
He said: "He was one of the senior British officials in charge of drawing up the dossier, a source of long standing well known to me."
The document in which the 45 minutes claim was made was drawn up by the Joint Intelligence Committee (JIC), made up of civil servants and the intelligence services, and presented to Downing Street.
Mr Gilligan said the 45 minutes claim was important because it went to the heart of the government's case that there was an immediate threat from Iraq.
He said his source told him: "I believe it is 30% likely that there was a CW (chemical weapons) programme in the six months before the war and more likely that there was a BW (biological weapons) programme, but it was small because they couldn't conceal a larger programme - the (UN) sanctions were actually quite effective, they did limit the programme."
And he said his source had been concerned that Downing Street had spoilt its case for action against Saddam by exaggerating the threat.
Tory MP Sir John Stanley told Mr Gilligan the committee had asked Foreign Secretary Jack Straw whether the wording of the 45 minutes claim in the dossier - published last September -was exactly the same as that provided in the intelligence services' assessment.
The MP said Mr Straw has told the committee in a written submission that "the same report was reflected in almost identical terms in the JIC's classified work. There were no further caveats used".
And Sir John asked if Mr Gilligan was saying the foreign secretary was lying, and accused the journalist of alleging that the JIC "connived in the embellishment of the dossier for political purposes".
Mr Gilligan replied that it was not his business to say if Mr Straw was lying, adding that it was his source, not him, who was alleging the dossier was changed.
Mr Gilligan said he asked his source how the dossier had been transformed.
"He answered with a single word which was 'Campbell' (referring to Alastair Campbell).
"I asked 'what do you mean, Campbell made it up?' He answered 'no, it was real information but it was included in the dossier against our wishes because it was not reliable, it was a single source and it was not reliable."
Mr Gilligan said his source had said that for weapons to be used within 45 minutes they would have had to be "relatively openly held".
"The likelihood is they would by now have been found," he said.
He said he doubted suggestions that Saddam may have destroyed or concealed weapons before the outbreak of war.
"I think it unlikely and illogical that Saddam, faced with an imminent threat to his regime and his very existence would give up his weapons immediately before a war was about to start."