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Advertising campaigns to raise awareness of the 2011 national censuses are being launched later.

The government measures the population of the UK every 10 years with a census in England and Wales, alongside separate surveys in Scotland and Northern Ireland.

The form asks for detailed information including nationality, religious faith and marital status.

Census Day, when the snapshot of the nation will be taken, is on 27 March.

The Office for National Statistics (ONS) runs the census in England and Wales. More than 35,000 jobs have been created for the duration of the exercise, as advisers, co-ordinators, enumerators and collectors gather information from 26 million homes.

The project is expected to cost £482m - more than double that of the last census.

In Scotland the census is run by the General Register Office for Scotland and overseen in Northern Ireland by the Statistics and Research Agency.

The campaigns for England and Wales and in Scotland are being unveiled later. In Belfast an event is being held to mark the start of Northern Ireland's census campaign, which was formally launched last week.

Online forms

Questions include national identity, ethnic group, educational qualifications, job titles, method of travelling to work and health status.

For the first time there will be enquiries on civil partnerships, second homes and recent migration.

But there will be no questions on income, sexual orientation or the nature of any disability.

The majority of questions are the same or similar throughout the UK, but the three surveys are customised to meet specific statistical needs.

It is compulsory to complete the census, and repeated failure to do so could result in a fine of up to £1,000.

Census director Glenn Watson said the data collected was invaluable to allow local and central government, businesses and voluntary groups to plan their service provision.

He also told the ����ý improvements had been made to try to avoid some of the problems experienced in 2001.

"We're producing a new address register to underpin this and we're providing an online completion option for the first time," he said.

"We're [also] providing a questionnaire tracking system which means we will know on a day-to-day basis who hasn't returned their questionnaire."

'True picture'

The ONS said the information gathered would remain confidential for 100 years, and would not be shared with other bodies.

An ONS spokesman said: "It is not linked in any way to any other government department. All your personal information is kept completely confidential for 100 years.

"There is no chance of this being linked to the tax people, the police or anything else like that, or even the immigration authorities.

"We are not targeting illegal immigrants, we only want to get a true picture of the population in March 2011."

But Daniel Hamilton, from campaign group Big Brother Watch, told the ����ý the census was a "very intrusive" way of simply duplicating information already available from sources like the electoral register, school records and tax returns.

He also said three million people had failed to fill out the 2001 form and some 300,000 had listed their religion as "Jedi".

"There's no reason to think that the British public will take this census more seriously than the last one," he added.

There is speculation the 2011 census for England and Wales may be the last in its current form, with future data gathered from records held by the Post Office, local government and credit-checking agencies.

An ONS spokesman said: "There is a working group within the ONS that is looking at a project known as Beyond 2011 and it is up to that group to make a report.

"The report will go to ministers and then it will go the government to decide on what the future of the census is. No decision has been taken yet."