Dabinderjit Singh says Sikh are being undermined
The UK Sikh Federation says it might take legal action over the Office for National Statistics' 2011 census form.
It says the ONS is breaking the law by putting a box for Sikhs to tick in the voluntary religion section of the form but not one in the ethnicity section.
It fears this will result in the number of Sikhs being underestimated and mean community services get fewer resources.
The ONS says space is limited on the form and putting the Sikh box under religion will generate the best data.
The census is a national survey that takes place every 10 years and the next one is on 27 March 2011.
Given the constraints in the questionnaire size and layout we can't put more than one tick box in for a particular ethnic or racial group
Peter Benton, deputy census director
It is a social, economic and racial snapshot of the nation and public bodies use the information to plan, fund and deliver services as well as allocate resources.
The ONS has included two new categories in the ethnicity section in next year's questionnaire. One is for Gypsy or Irish Traveller and the other is for Arab.
But Sikhs argue that if they are not represented in this section then they will effectively remain invisible to the authorities.
"Basically, as far as the Sikhs are concerned, the questions that the census asks are being asked in a particular way so that it's actually undermining the Sikh community and the services we're entitled to," said Dabinderjit Singh, an adviser to the federation.
"At the last census in 2001 the number of Sikhs that we believe were recorded in the census were actually half the number of Sikhs in the UK.
"Therefore, policy makers, whether it be central government or local government, have been making decisions on the basis of incorrect information."
He believes that the fact that UK law recognises Sikhs as a separate race gives them a "very clear basis" for taking legal action.
The previous census in 2001 suggested that there were 336,000 Sikhs in Britain whereas campaigners claim the true number is closer to 600,000.
The 2001 census was the first time that the form had a section on religious beliefs.
And, although this section included an option for Sikhs, it was the only question that was voluntary, with households having to answer everything else.
The ONS believes that having a Sikh box in the religious identity section is adequate and that it is not necessary to have a box to tick for Sikhs in the section on ethnicity.
Deputy census director Peter Benton: "Given the constraints in the questionnaire size and layout we can't put more than one tick box in for a particular ethnic or racial group.
"We have chosen to put it in the religion question as that gives the best quality data."
The ONS said it had effectively finished putting together the census form and that it planned to start printing the questionnaires in a few months.