Proposals for the content of the next census have faced criticism from some quarters over tick-box options on ethnicity and national identity.

British Sikhs condemned the Office for National Statistics (ONS), which worked on the 2021 survey plans, for being "secretive", while Cornish nationalists said the proposals were "illogical, prejudicial, disrespectful".

The Sikh Federation UK has pledged to campaign for an ethnic tick box in the next census, while the leader of Mebyon Kernow, the Cornish party, called for inclusion in the national identity section.

The next survey of households in England and Wales is due to take place on March 21 2021, subject to parliamentary approval.

Government plans showed that, while Sikh remains an option in the faith section, it is not being considered for inclusion in the ethnicity section of the census.

The Sikh Federation said it is prepared to take legal action against the ONS claiming the organisation had become "secretive and focused on finding reasons not to grant Sikhs an ethnic tick box".

The chairman of federation, which describes itself as the biggest Sikh campaigning organisation in the UK, said the group believes it can gain support from enough MPs to ensure the option is included.

The white paper published by the Cabinet Office on Friday stated that there are different views on that matter within the Sikh community and that there was evidence that having a specific response option in the ethnic group section "would not be acceptable to a proportion of the Sikh population".

The paper added that "views on each side are passionately held".

The ONS said its consideration of the ethnicity option included conducting focus groups with more than 50 participants from Leicester, Birmingham and London.

Among the responses they said a "small number" of older, male respondents favoured the ethnicity Sikh tick-box while younger second-generation participants wanted to express their Sikh background through the religion question.

Federation chairman Bhai Amrik Singh said: "ONS has gone to great lengths to maintain the status quo and continue with reinforcing the institutionalised discrimination of Sikhs by public bodies.

"We are aware of some of the pressures that have been exerted on them.

"However, these are proposals and the final decision on the census questions rests with Parliament."

He added: "We are confident we can secure the support of up to 350 MPs from across the political spectrum, including ministers and shadow ministers to back the Sikh community and we will be successful in securing an amendment to the Census Order when it is presented to Parliament in autumn 2019."

The white paper said the need for a Cornish tick-box was too localised and "not strong enough to justify its inclusion in the nationwide census".

Councillor Dick Cole, leader of Mebyon Kernow, tweeted: "This is illogical, prejudicial, disrespectful and just plain wrong. We must fight this and win a #CornishTickBox."

In what is billed as a "digital-first" census, up to three quarters of respondents to the 2021 survey are expected to complete the questionnaire on the internet.

By Aine Fox