Britain's biggest Census which will quiz people on who stays overnight and about civil partnerships came under fire from critics of state snooping yesterday.

They attacked organisers of the £482million survey for wasting time and money as an advertising drive was launched in the run-up to Census day on March 27.

For the first time the Census includes questions about civil partnerships, second homes and when migrants arrived and intend to leave.

Householders must also name guests, such as boyfriends and girlfriends who stay the night of March 27, including their age, sex and usual address.

It is compulsory to fill in the census or face a fine of up to £1,000. The operation will involve 35,000 field staff, who will gather information from about 26 million households.

As well as relationships, the 43-question survey covers areas ranging from health to fluency in English, type of central heating, how people travel to work, job titles, educational qualifications and work addresses.

Religion will remain the only non-compulsory question. In the last census in 2001, 400,000 declared they were Jedi Knights.

Also for the first time, householders will be able to fill in their forms online.

Organisers say data from the survey, carried out every 10 years, is vital to help councils and health authorities plan services.

But Daniel Hamilton, campaign director of civil liberties group Big Brother Watch, said: "At 32 pages, the census includes intrusive questions on your proficiency in English, your health, when you last worked and the type of central heating you have.

"The Government has no need, and no right, to know this about you. It is also a monumental waste of time and money.

"A large number of the questions duplicate data already held by the authorities on databases such as the electoral register, school records, tax returns and GP information."

He also condemned the "entirely hollow but nevertheless bullying" threat of fines.

In 2001, 3 million people refused to fill in the forms but there were fewer than 40 convictions.

A spokesman for the 2011 Census denied it was intrusive, saying there were no questions on income, the nature of disabilities or sexual orientation. Asking people if they were in same-sex civil partnerships was about marital status not sexuality, he said, adding: "The questions have been tested on over 100,000 people and the overwhelming majority found the questions were acceptable."

An Office for National Statistics spokesman said: "All personal information is kept completely confidential for 100 years.

"There is no chance of this being linked to the tax people, or even immigration authorities. We only want to get a true picture of the population in March 2011." The Census covers England and Wales, with separate surveys in Scotland and Northern Ireland.

The £482million cost is more than double that of the last Census. But this one could also be the last of its kind as an ONS working group is looking at its future. In future, information could be drawn from Post Office records, councils or credit reference agencies instead.

Yesterday a campaign bus was due to begin a tour of Britain to promote the Census.