Census 2011: Cardiff council fears 22,000 'undercount'
Cardiff council's leader said he believes there was an "undercount" of 22,000 in the city's population at the last Census.
Rodney Berman told the BBC Politics Show on Census Day the city may have lost out on £85m funding since 2001.
Meanwhile, Muslim leaders also encouraged followers to declare their religion because it is "important to be counted".
A Census spokesman said it was "important" people returned 2011 forms.
Mr Berman said the information was crucial for the provision and funding of public services.
The Welsh capital, Wales's largest council area, had an estimated population of around 305,000 in 2001.
"We believe there was an undercount of Cardiff's population, we think by about 22,000," said Mr Berman.
"That's had a major impact because the funding we've had since then is less than we think we should have got.
"The funding we get is based on the number of people we have living in the city and we reckon that (undercount) could have cost us £85m pounds in the intervening years when all added together.
"That's quite an impact we have to concern ourselves with".
Meanwhile, secretary general of the Muslim Council of Wales Saleem Kidwai said he thinks thousands of Muslims in Cardiff did not name their religion in the 2001 census due to their suspicions of the process.
The 2001 census found 11,261 Muslims in Cardiff. Mr Kidwai, said he believed the figure was closer to 20,000.
A census spokesman said the question (No 20) about religion was voluntary.
The 2001 census was the first time that the form had a section on religious beliefs.
The Office for National Statistics (ONS), who organise the Census, included two new categories in the ethnicity section in the 2011 questionnaire.
Mr Kidwai said Muslims should complete the census because it was legal requirement to do so and also so that the Muslim community could be assessed for resource allocation.
He said many Muslims did not answer the question last time as it was a voluntary question and also they may have been apprehensive about why the authorities wanted to know what their religion was.
Mr Kidwai estimates Wales has around 60,000 Muslims and is urging them to fill in both the ethnicity question and the religion question.
He said: "It is important to be counted. The resources the community depends on are affected by the census.
"It is will be good for the community and the country if we can see 'this is the number of people', 'these are the problems' and 'this is how we can deal with it'."
An ONS spokesman said the question on religion was included "for a number of reasons".
He said: "The census is used as a basis to fund a lot of services and increasingly these days a lot hangs on people's religion.
'Very, very expensive'
"The Muslim community has requirements for the allocation of resources and services which are unique to them in the same way that the Christian community might have requirements for different kinds of services to their local community. "
Pete Benton, the deputy director of ONS, said it was important to get it right.
"The census provides vital information for planning everybody's local services, whether it's care services, school places or even how wide your roads needs to be."
However, Guy Herbert from the No2ID campaign believes the Census is not cost-effective.
"This is a very, very expensive way of doing it," he said.
"The present Cabinet Office minister, Francis Maude, said last summer there must be cheaper, quicker and more up to date ways of doing it. (I believe) a snapshot every 10 years for £480m is a waste of public money".
People have been urged to return their Census forms as quickly as possible, and certainly within 10 days.