By Peter Griffiths

LONDON - The opposition Conservatives narrowly extended their lead over the ruling Labour Party in the opening days of the British election race, but polls on Sunday cast doubt on whether they will return to power after 13 years.

The Conservatives lead over Prime Minister Gordon Brown's ruling Labour is too small to secure overall control of parliament at the May 6 ballot, two surveys suggested.

While David Cameron's centre-right party would be the biggest in the lower chamber, it would not have a majority.

The prospect of a 'hung parliament' has worried some investors, who fear it will hamper decision-making and make it harder for the new government to take steps to reduce Britain's record budget deficit.

Most recent polls have pointed to an inconclusive victory for the Conservatives, whose poll lead was as high as 20 percent last year.

Three polls in Sunday papers suggested Brown's centre-left party is trailing the Conservatives by between seven and eight percentage points after debate over taxation and the economy dominated the start of the campaign.

A ComRes survey for the Independent on Sunday and the Sunday Mirror put the Conservatives up two points at 39 percent, with Labour at 32 percent.

A YouGov/Sunday Telegraph poll gave the Conservatives 38 percent, up four, with Labour down three at 30 percent.

That would probably leave the Conservatives 25 seats short of a majority in a parliament, the Telegraph cited politics professor John Curtice as saying.

However, if their share of the vote was stronger in marginal seats, the party could still win with a small majority, he added.

A third poll for the Sunday Times also suggested the Conservatives might scrape a small majority.

It put them at 40 percent, up one point, with Labour up three points at 32 percent.

Most analysts say the Conservatives need a nine or 10 point lead over Labour to secure a majority in parliament, although there is no exact target.

The result is based on the party that wins the majority of electoral districts, rather than by looking at the overall share of the vote each side wins.

The electoral map is often seen as being biased against the Conservatives.

Britain's third biggest party, the centrist Liberal Democrats, who could hold the balance of power in a hung parliament, lost support in two of the polls.

They have between 16 and 21 percent of voters' support.