The Christian Institute has condemned the government's latest plans for independent schools, saying they will curtail freedom of speech.

Under new guidelines, teachers working in independent schools – including Christian schools and academies – will have to actively promote 'rights' in every subject.

The Christian Institute says this could be abused to mean pupils are not taught about Christmas and other religious subjects, for fear of causing offence, and teaching on sensitive issues such as marriage would be under close scrutiny.

The new guidelines follow the discovery of the Trojan Horse scandal in Birmingham – an alleged plot by Islamic extremists to take over a group of schools and promote fundamentalist ideology. In the aftermath of the discovery, newly appointed Education Secretary Nicky Morgan declared that all schools will be expected to promote "fundamental British values".

A six-week consultation due to end next week has given schools the opportunity to respond to the guidelines, but as it is currently the school holidays the Christian Institute has deemed it insufficient, and indeed contrary to government rules, and is now therefore threatening judicial proceedings.

A spokesperson for the Department for Education has stressed that the new Independent School Standards "are designed to ensure every school prepared children for life in modern Britain."

"We make no apology for demanding high standards and the promotion of tolerance and respect of all faiths and cultures," she added.

However, Chief Executive of the Christian Institute Colin Hart has warned that the plans are deeply "flawed and unlawful".

Hart told Christian Today that the guidelines and "so broad" and "there is so much imprecision" that they could be used to stop teachers using gender specific terms such as husband and wife, or even mother or father.

A press release from the group says Christian schools could be forced to "downgrade religious festivals such as Christmas, so as not to offend atheists and those of other faiths."

This will have "serious and perverse consequences," the statement warns.

"The danger is that this will lead to more intolerance," Hart explained.

"Atheism is not neutral – some people think it is, but secularism is not neutral at all, it is intolerant towards religious belief. We have to have freedom, and the great danger with these equality and diversity laws is that they will cause more intolerance."

He described the government's new guidelines as an "extraordinary overreaction" to the Trojan Horse controversy. In a statement from the Christian Institute he added: "This is a classic case of the Government overreacting to a perceived problem. They are shocking in their breadth and range and would destroy the independent sector.

"They mistakenly advance the principle that political correctness equals British values. Accordingly they could be used to punish any school in the independent sector which has a religious ethos, a set of traditional beliefs, or who don't over promote every minority group's world view."

Hart also said that "private schools, academies and free schools would have much less control over their ethos than ordinary state schools" and accused the government of pursuing "an intolerant and aggressive agenda".

He has therefore welcomed concerns raised by other organisations including the Independent Schools' Council and the Association of School and College Leaders.

The Department for Education Spokeswoman responded: "It is simply untrue to say that the proposed changes - which received 1,400 responses in the last six weeks - would prevent teachers using gender-specific terms or require schools to downgrade Christian festivals.

"We have received a letter from the Christian Institute's legal representatives and are considering our response."