By John Andrew BBC local government correspondent

The BNP's surprise victory in Blackburn brings their national tally of council seats to four - all of them in Lancashire.

They won three other seats last May in nearby Burnley, one of several North West towns where racial tensions ended in rioting last year.

What is surprising about the Blackburn result is that race relations there are generally seen as more harmonious.

Indeed, the Mill Hill ward where the BNP won is considered a well-integrated area with many Asian and white families living side by side.

The BNP victory... is another worrying signal for mainstream parties

Ibrahim Masters, who chairs the Lancashire Council of Mosques, said Blackburn had been hailed as a good example of a multi-cultural society and had pulled together when other towns were affected by riots.

The foreign secretary and local MP Jack Straw, who had helped in the Labour campaign, noted that more people voted Labour, Lib Dem or Tory than BNP by a margin of more than two to one and said the result would not obstruct efforts to build a more tolerant community.

Bridgehead

The BNP victory, though, is another worrying signal for mainstream parties that the far right party can successfully exploit fears over issues like asylum seekers and immigration.

Its policies include ending all immigration and introducing a scheme of voluntary repatriation.

It is true that four council seats does not add up to more than a minor toe-hold in local government, but the BNP leader Nick Griffin sees this as a bridgehead from which the party can build more support.

They are already targeting more seats in Lancashire.

It is important too to look beyond just the seats they have won.

Last May, for instance, they averaged 20% of the vote in the seats they fought - their best performance since the 1970s.