The report will renew worries about voter turnout

Harriet Harman was speaking after the elections watchdog revealed up to 3.7m people eligible to vote are not registered to go to the polls.

The Electoral Commission says the findings show the need to shake up the way people register to vote.

But Ms Harman said any changes must not make the problem any worse.

She pointed to one ward in Liverpool where low registration rates were coupled with a 30% election turnout.

"We have really got what I am describing as democracy deserts - a situation where democracy is not operating," she told BBC Radio 4's World At One.

Appeal for caution

The head of each household currently registers people to vote.

The Electoral Commission wants a new system of individual registration, where each person registers to vote themselves - allowing new safeguards against fraud.

But Ms Harman said such a move could see a further drop in registration numbers - as had happened in Northern Ireland.

ETHNICITY DIFFERENCES 17% of ethnic minority groups overall unregistered 6% of white people unregistered 7% Asian people (Indian, Pakistani, Bangladeshi) unregistered 9% black Caribbean people unregistered 24%-37% black Africans, other black groups, Chinese people and those in other ethnic minority groups unregistered Web and text vote trials dropped

"We must be careful that we take the appropriate measures to make the process fraud proof and secure and not make the problem worse," she added.

The Electoral Commission's report was compiled using research from the Office for National Statistics, a Mori poll and evidence from eight local council areas.

It is the first estimate of registration rates since 1993.

It says between 8% and 9% of those eligible were not on the electoral register in 2000.

Nearly half of all those not registered were either living with parents, home movers or people renting privately.

Race differences

Young people were the most likely to be unregistered - including an estimated 16% of 18-24-year-olds.

Men were slightly less likely to be registered than women.

And people from ethnic minority groups were about three times more likely to unregistered as white people (17% compared with 6%) but with wide variations.

Unemployed people were more likely to be unregistered than those in work.

But the Electoral Commission is particularly concerned about findings that suggest "the head of the household" was much more likely to be registered than other eligible people in their home.

And people are less likely to be registered if they are not related to the household head.

The watchdog's chairman, Sam Younger, said registration was the lynchpin of the elections system.

"Registration should be a matter for each individual rather than for heads of household," he said.

"It is vital that a system of individual registration is introduced as soon as possible so that the register used at the next general election is secure, accurate and commands confidence."

Modernisation

The government plans to publish a bill on election rules in the near future.

Ministers have already proposed having one registration form per household which is signed by everyone registering to vote.

But Conservative shadow constitutional affairs secretary Oliver Heald said: "The government's obsession with electoral 'modernisation' has compromised Britain's traditional reputation for free and fair elections, and undermined both the integrity of the system and public confidence in it."