Lawyers for the Sikh Federation have written to ministers warning them that it would be unlawful not to include a Sikh ethnicity tickbox on the form, as the Office of National Statistics (ONS) recommended last year.

In 2011, the census recorded about 430,000 Sikhs on the basis of a question about religion. The federation estimates there are some 700,000 to 800,000 Sikhs in Britain.

Rosa Curling, of law firm Leigh Day, which represents the Sikhs, said: “Our clients have identified a number of flaws within the process for determining whether to include a Sikh ethnicity category in the 2021 Census.

“They believe it is crucial that individuals are able to identify as ethnically Sikh in the next census to ensure a more accurate picture of the community is taken. This will ensure public bodies are fulfilling their duties under the Equality Act when making decisions about the allocation of vital public services.”

A spokesperson for the ONS said: “No group will be missed out in the digital-first 2021 Census. The ONS recommendations follow extensive research and consultation with groups and individuals and everyone who wishes to identify as Sikh will be able to do so.

“The religion question will have a specific Sikh tickbox response option and everyone who wishes to identify as Sikh in response to the ethnicity question will be able to do so through a write-in option. We believe these proposals will enable public bodies to work with the Sikh community in shaping public services to meet their needs.”

However, not all Sikhs recognise the group as a separate ethnic group.

Lord Singh of Wimbledon told the Times of India: “Sadly, no one in the unrepresentative SFUK-run All Party Parliamentary Group for British Sikhs has taken up the challenge to debate their flawed and rejected Sikh ethnic box campaign. For a start we’d like to educate them on Sikh teachings.

“First lesson – Guru Nanak was the founder of a major world faith, not an ethnic group.”