Has the trust gone?

By Nick Assinder BBC News Online political correspondent

It is just possible that Labour will look back at the sensational Brent East by-election as the turning point for this government.

It could well be a defining moment for Tony Blair who has been delivered the clearest possible message about what his own voters think about him.

Tipping point for Tony Blair?

Certainly Iain Duncan Smith will undoubtedly repeat that it appears they don't trust a word Mr Blair says anymore.

As Charles Kennedy was celebrating an extraordinary victory, Tony Blair was left reeling from a substantial blow to his image as a prime minister the electorate feels it can trust.

It was the issue on which he built his leadership of the Labour party and then his premiership. And it could not be clearer that, after his most difficult year yet, that support is very thin on the ground.

Downward slide?

The issue was raised time and again on the doorsteps of Brent which, until Friday morning, was considered rock solid Labour territory.

The culture of spin, the Iraq war, the David Kelly affair and continuing rows over public service reform and abandoned manifesto promises have been eating away at the government for months.

There is always a point with any government which, when viewed in retrospect, is seen as the point where all the negatives combined to set it on the path to failure Blair's by-election gloom

Attempts to put spin behind him have so far failed to stop the rot. And the revelations around the Kelly affair and the way he took Britain into war appear to have intensified the feelings of mistrust.

Whether or not this really is the start of a downward slide for Tony Blair is difficult to predict, however.

There is always a point with any government which, when viewed in retrospect, is seen as the point where all the negatives combined to set it on the path to failure.

Backbench critics

Similarly there is often a point when backbench MPs start to fear that their leader has become a liability rather than an asset.

That judgement is always sharpened when those MPs believe their own seats are at risk in a future general election.

Still the official opposition?

It happened to Margaret Thatcher - one of the Tories' most successful leaders ever - and the party lost no time in dumping her before going on to surprise everyone by winning the next election under John Major.

Again, it is too soon to suggest this is the point at which Labour MPs may decide their once-infallible leader is now dragging them down.

But it is just possible.

Some of his fiercest backbench critics like Glenda Jackson have seized on the by-election result to renew their demands for him to resign.

'Mammoth task'

But there is little sign yet that is a widespread demand on the Labour benches.

And the prime minister does have time on his side - but not that much.

In the two years or so between now and the next general election he has a mammoth task ahead of him.

He has to regain the trust he has patently lost, he has to show he is listening to concerns over his more controversial policies, and he has to persuade his own disillusioned supporters that he still has something special to offer them.

This may be just a single by-election, but it may yet go down in the history books as the poll that shook the government.