A warning to the main parties: every vote is up for grabs

It has been quite some time since Labour has been able to take comfort from a by-election result. And this week's twin contests in south London and south Wales will have done nothing to change that trend. Labour's candidate was again rejected in Blaenau Gwent. And in Bromley and Chislehurst the party was pushed into fourth place, behind the UK Independence Party.

Even allowing for the tendencies of by-elections to go against ruling parties, these were terrible results for Labour. It is often said that electorates punish divided parties. It now seems that they are punishing a divided leadership. The power struggle between Tony Blair and Gordon Brown has eroded Labour's credibility among voters. The air of perpetual crisis around the Government has also damaged its image. The cash-for-peerages scandal, John Prescott's humiliation and a succession of Home Office blunders have taken their toll.

Yet the Conservatives can take no comfort from these results either. The fact that Bromley almost slipped from their grasp shows how much the Conservative Party still has to accomplish to win power. David Cameron's leadership has been so lauded of late that the Conservatives might have expected to romp home. In the end, they held on only by the skin of their teeth. The Conservative chairman Francis Maude was essentially correct yesterday when he argued that these results show the need to drive change "faster, wider and deeper". It is not enough for Mr Cameron to be favourably regarded - the party must be trusted too.

Strangely, the present political climate probably did the Conservatives no favours. Mr Cameron has spent the past few weeks talking about some very traditional Tory topics: law and order, Europe, the Human Rights Act. Many of his interventions could have been delivered by Iain Duncan Smith or Michael Howard. Mr Cameron should be pressing the Government on the environment, social issues and civil liberties, where a strong opposition voice is needed more than ever. What this week's result underlines is that the old Tory obsessions turn voters off.

The Liberal Democrats are the only mainstream party with anything to celebrate from these by-elections. Despite Sir Menzies Campbell's lacklustre leadership, the party he leads is still, evidently, an election-winning machine. And by-elections are a particular speciality. An impressive surge slashed the Tory majority in Bromley from 13,342 to 633 on Thursday. And this comes only five months after the shock victory over Labour in the Dunfermline and West Fife by-election. The Liberal Democrats remain a potent threat to both Labour and the Conservatives. Three-party politics is still very much with us.

But so too are the independents. In the 2005 election, Blaenau Gwent voters rejected the official Labour candidate in favour of a local councillor who had resigned from the party over the imposition of an all-female shortlist. This week, instead of flocking back to Labour, voters elected another independent.

This should be a warning for all three mainstream parties. If a strong local independent candidate emerges, with a serious campaign, voters are increasingly likely to support them. Nor is this a passing phenomenon. As we saw in Wyre Forest in 2005 - and now in Gwent - independent seats can quite easily remain independent.

Labour, the Conservatives and even the Liberal Democrats should beware. The idea that the public has nowhere else to go is quite wrong. In this age of increasing consumer choice and electoral promiscuity, every vote is up for grabs.