Almost £500 million will be spent on the biggest and most expensive census in an attempt to measure Britain's true immigrant population.

Questions have been added to try to establish how long people have been living here, what passports they hold and their nationality.

Ministers have been forced to admit that they have no idea how many immigrants are in Britain, which means that public services in areas of high migrant populations are often stretched because spending decisions are made using inaccurate data.

For the 2011 Census, millions of pounds will be spent on a new mailing list to make sure that every property is included.

The Office for National Statistics (ONS) will also hire about 35,000 temporary staff to track down people who fail to fill out their form. The full cost of the operation will be £480 million compared with £210 million in 2001.

Critics were unconvinced, however, that the extra money would make any difference. They say that the census is simply outdated as a vehicle for gathering comprehensive and up-to-date information, and unpublished data already held by government departments should be used instead.

Announcing plans for the census, which will take place on March 27, 2011, officials said they would devote considerable resources to getting back as many returns as possible.

“Everyone who does not fill in the census will receive a knock on the door. We will have teams of 12-15 people who will visit households who have not returned their form and help them fill it in,” said Glen Watson, the census director.

Mr Watson said it was an offence punishable by a £1,000 fine not to fill in a census and hundreds of people have been taken to court in the past for refusing to complete the form.

However, a bigger problem is the far larger group who cannot be traced. In 2001 the ONS failed to find about one million people and were forced to extrapolate information from other returns. In addition, the first data is not published until about 18 months after census day, making it already out of date.

James Hulme, a spokesman for the New Local Government Network (NLGN), an independent think-tank, said: “We calculated that the Government could save about £250 million by drawing on records from existing public services, such as GP surgery lists and the electoral roll which is updated each year, rather than every ten years. At a time when the public finances are stretched, this is an ideal way of saving money.”

Councils said that they did not want spending decisions to be made on the basis of the census. They were furious at being short-changed because resources were allocated on the basis of the 2001 Census.The Local Government Association said that improving alternative sources of information such as national insurance numbers would give a much clearer picture.

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