By Nick Assinder BBC News Online political correspondent

Campbell attacked media "liars"

That is so often the way with the current government.

And without a killer fact to blow one side or the other out of the water there was probably nowhere else it could go.

Tony Blair's chief spin doctor, his director of communications Alastair Campbell, could not have made that more clear during his three hours of evidence to the Commons foreign affairs committee.

And in doing so he put his own credibility - and probably his job - on the line.

Straw said 'dodgy dossier' was 'a complete Horlicks'

In a robust and sometimes exasperated and angry performance in a packed committee room he bluntly stated this was now a matter of believing either the prime minister, the heads of the intelligence services and himself - or those accusing them of conning the country into war.

He made no bones about the seriousness of the allegations but went on to claim that they were based on a "storm in a teacup" rather than a "Horlicks", as claimed by Foreign Secretary Jack Straw.

Bated breath

But if he said it once he said it a dozen times - his critics, including the BBC, were lying.

Yes, an official had made a mistake in incorporating bits of a magazine article into the so-called "dodgy dossier".

But it had been admitted to as soon as it was discovered and, in any case, mattered little because the facts in the document were accurate.

What he did not say, but what now underpins this entire row, is that if his critics are not lying, then certainly he and probably the prime minister are. It's as simple and as serious as that.

By the end of it he and the prime minister have reason to feel more rather than less relaxed about this issue.

This was the performance the whole of Westminster had been waiting for with bated breath since this row erupted.

It is only the second time Mr Campbell has appeared before a committee of MPs and he only came, he insisted, because he was infuriated with the lies being pedalled about him, the prime minister and the intelligence services.

Utter contempt

It was a shirt-sleeved performance which saw Mr Campbell starting clearly tense, and ending far more relaxed.

In between he ran the gamut from apologetic to dismissive, frustrated to exasperated and, quite regularly, table-jabbing angry.

There was a sort-of-threat that the BBC had better apologise quickly or ..... and he left that hanging.

And he certainly did little to disguise his utter contempt for some groups of journalists - particularly the ones he said were waiting to write "blah blah blah, how did he do" rather than cover the details of the event.

The one emotion he probably missed was defensive, but that was never his style.

Finally there were the lip curling put downs of certain journalists that anyone who has attended a Campbell briefing recognised instantly.

MPs to decide

By the end of it - and here comes the "blah, blah, blah" - he and the prime minister have reason to feel more rather than less relaxed about this issue.

The allegations are certainly the most serious that can be levelled at a government and its officers.

And there could not have been a more comprehensive denial of the claims.

But, needless to say, much of it came down to assertion.

And it will now be, in part, up to this committee of MPs to decide who is telling the truth.

If they back Mr Campbell then much of the wider criticism over the government's handling of the conflict will evaporate.

The government's critics will find it hugely difficult to press ahead with their case.

If, however, this committee suggests they do not believe Mr Campbell's account of events the consequences will be incalculable.