The next census will be the last of its kind under plans being drawn up for its replacement with a national population register.

The £500m census on March 27, 2011, will be the earliest in the year since the first Britain-wide survey in 1801, to avoid Easter and the Scottish Parliament elections, and would be the final such 10-yearly head count if the plans under preparation are backed by ministers in the next few months.

At Holyrood and Westminster, which have separate control of the census north and south of the border, there is pressure growing for a replacement.

The New Local Government Network, a London-based think tank, warned yesterday that it is out of date as soon as it is published, 20 months after the count takes place, and it under-estimates the number of people living in Britain. Yet it still costs £500m across Britain - £60m in Scotland.

The group claims there is poor quality information on households, high rates of population mobility and a growing reluctance to fill in official forms, with a claim that as many as 900,000 people could have gone uncounted from the 2001 census - 100,000 of them young men.

Chris Leslie, the NGLN director, commented: "The census has been around for 200 years and it is no longer gathering the right sort of data for modern services. Not only does central government not know where it should distribute public money, but local councils do not have the information or flexibility to work out where best to spend money to tackle worklessness and crime, or to gauge where future demand will be for care homes and schools."

In Scotland, Registrar General Duncan Macniven is drawing up plans for a different way of counting the population. He claims the census is accurate, and that non-compliance is low, at around 4%, but that pressure for change is coming from user groups. They include those who run public services such as health, education and police.

In an interview with The Herald, Mr Macniven explained replacement of the census will require an identity register, which could be combined with an address register which has recently been drawn up for the whole of Scotland by local councils.

The identity register risks becoming enmeshed with the controversy over Labour plans to introduce identity cards. But Mr Macniven claims there are other ways such a register could be compiled, and at lower cost. His staff are preparing the ground for the NHS central register of GP patients to be used as "the spine" of an alternative census.

But the Registrar-General stresses he is independent of government, and there has been an assurance of Whitehall ministers that the data collected will not be used for the identity card system.

He said: "There needs to be careful thought given on whether we're better to stick to the census after 2011 or whether we are better to move to an alternative system. The decision is for the Scottish Parliament."