Census dawdlers urged to use online form
People who have still not returned their completed census form are being encouraged to fill it out online to avoid a home visit from a collector.
On Wednesday, 30,000 census collectors in England and Wales will begin knocking on doors of households who fail to meet Monday evening's deadline.
Completing the census is compulsory; not doing so could mean a £1,000 fine.
Every UK home was sent a survey, which aims to get a snapshot of how people in the UK live their lives.
It asks who lives in the home, and details about jobs, education and ethnic background.
Census organisers say their collectors will help people with difficulties filling out the compulsory questionnaire and supply replacement questionnaires if the original was lost or damaged.
This year for the first time households have been able to complete them online.
Census director Glen Watson said completing the form on time and online kept costs down.
Within three days of Census Day - 27 March - more than half of all forms in England and Wales had been returned. Northern Ireland had a similar response.
In Scotland, which has not released any figures yet, census organisers are sending letters to about 200 householders who are refusing to take part, warning them they face a £1,000 fine.
The government says the census, conducted once every 10 years, enables it to plan public services for the future.
Questions include national identity, ethnic group, educational qualifications, job titles, method of travelling to work and health status.
For the first time there are inquiries on civil partnerships, second homes and recent migration.