A bizarre collection of items including passports, cheques, death certificates and even a drawing of the Queen have been enclosed with completed 2011 census forms, the Office for National Statistics has said.
The body said where possible it is returning the items, but it appealed for people not to send extra documentation with their completed questionnaires.
Organisers suggested that those people who have not yet completed the census should do so online for greater convenience.
Other items included by people include credit cards, a set of deeds, family photographs and a completed driving licence application form.
Census director Glen Watson said: "I'm delighted that so many people have sent their questionnaires back promptly, but urge those who are returning their questionnaire this week to check carefully that they're not including their shopping list."
Census collectors began the task last week of knocking on the doors of an estimated seven million households in England and Wales which have failed to fill in and return the forms.
Around 26 million households in England and Wales were sent the questionnaire last month to fill out on March 27, or as soon as possible afterwards. Nearly three-quarters of households so far have completed and returned the forms.
Filling out the census is compulsory, with the threat of a fine of up to £1,000 if a questionnaire is not completed and returned.