Seven million households which have not completed and returned the census in England and Wales are being warned that they face a visit from officials.
Some 29,000 census collectors will begin visiting householders who have not returned their questionnaire to help or persuade them to complete it.
Completing the census is compulsory and 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 live their lives.
About 26m households in England and Wales were sent the questionnaire.
Those who received the forms had to fill them in on 27 March or as soon as possible afterwards.
The Office for National Statistics (ONS) said legal action will only be taken against people who persistently refuse to complete a questionnaire.
Nearly three-quarters of households have completed and returned the forms.
The north east of England has seen 75% compliance, compared to London which has the lowest compliance rate at just 60%.
The ONS said it had anticipated that London would be a "challenging area" and almost 6,000 collectors would be door-knocking across the city.
Glen Watson, 2011 census director, said he was "pleased that nearly three-quarters of the forms have come back just nine days after census day".
He said collectors would start visiting households from 1400 BST on Wednesday to collect questionnaires.
The ONS said all 2011 census field staff will be wearing photo identification badges and carrying other "visibly branded" items.
Anyone unsure about the identity of anyone claiming to be from the census should ask to see their proof of identity before answering any questions.
Those who are still concerned can contact the census helpline on 0300 0201101 in England or the Welsh helpline on 0300 0201130.