The Conservative government recognized that changing the mandatory long-form census into a voluntary survey would make some data unusable for federal institutions, according to documents submitted Monday in Federal Court.

The documents were presented as evidence in a court challenge against the government led by a group that represents minority francophone communities across the country.

A lawyer representing the Federation of Francophone and Acadian Communities addressed a federal court in Ottawa on Monday, arguing that the switch violates the Official Languages Act.

The group wants the decision to be subject to a judicial review.

The document, obtained by the group for the court case, looks at how the voluntary survey will affect the data collected by Statistics Canada.

"It is recognized that the quality of the data collected by the voluntary (survey) will be lower than that of a mandatory survey," it reads.

Some survey data, "will not be useable for a range of objectives for which the census information would be needed," wrote Rosemary Bander, the assistant chief statistician.

The government tried to address some of the group's concerns by adding more questions about official languages in the mandatory short census, but the group said a simple headcount wouldn't suffice.

"The government must know what our needs are terms of health, of education, of everything, to know how to better deliver services and programs and respond to our needs," said the group's president, Marie-France Kenney.

"Our needs are very different from one end of the country to the next and often our communities are scattered and very small."

One of the many arguments against the Conservatives' plan is that a voluntary census won't accurately track the status of the French language in Canada.

The Official Languages Act directs the government to take actions that will help the development of linguistic minorities and enhance their vitality.

For his part, Prime Minister Stephen Harper and the Conservatives maintain that the mandatory long-form census is intrusive and the threat of jail time for non-compliance is unacceptable to Canadians.

The Conservatives' controversial plan has triggered opposition from groups across a wide political spectrum, including Statistics Canada. The former head of the agency stepped down from his role after the government claimed he had given his blessing for the move to a voluntary census.

The lawyer representing the francophone group on Monday said the first week of October would be the last opportunity for StatsCan to change its census process.

A number of groups are voicing their opinions on both sides of the argument, in relation to the long-form census issue.

The census debate also continues in Parliament.

The Liberals are expected to submit a private member's bill to write the long-form census into law by the end of the week. They also have an opposition day motion set for debate on Tuesday.

With files from The Canadian Press