Christians who oppose gay marriage could be targeted with "false accusations" of extremism under an upcoming bill, a lobby group has warned.

David Cameron will introduce measures to ban organisations, silence individuals and close down groups that "promote hatred" in legislation to be announced later this month. The bill will be the central focus of the Queen's Speech on May 18, according to The Times.

However Simon Calvert, deputy director of public affairs at The Christian Institute, told Christian Today the new powers could be used to target Christians

"The government talks about tackling extremism and preventing people promoting hatred," said Calvert.

"If those words had their ordinary meaning Christians would have nothing to worry about but unfortunately they don't. People routinely use the word 'extremist' to label Christians and they often falsely accuse them of hatred.

"If you put those two words into a statute how can you be sure they won't be interpreted in that same overly broad way to capture innocent Christians simply going about the business of preaching the gospel and declaring the council of God?"

A home office source, quoted in The Times, admitted: "Getting agreement about the thresholds for what constitutes extremism and what needs to be protected as free speech is not going to be easy or straightforward."

For Calvert, the problem in the government's approach is its "over emphasis on what it calls non-violent extremism".

He told Christian Today: "Sadly when the government says it wants to promote British Values it seems to mean gay rights.

"Trying to force Christians to sign up to LGBT rights won't do anything to stop Islamist terrorists murdering innocent civilians."

Most details of the bill are not yet known but it is understood the controversial extremist disruption orders (EDOs) will be a central aspect.

The Christian Institute has formed an unlikely campaign group with the National Secular Society and the Peter Tatchell Foundation to oppose EDOs which Calvert says would threaten free speech. "We are united in our belief that free speech is a vital civil liberty and must be protected," he said at the launch of their Defend Free Speech campaign group. "This legislation is badly conceived and will be bad for society."

Gay rights campaigner Peter Tatchell said: "Heavy-handed legal restrictions and sanctions on free speech undermine the democratic, liberal values that extremists oppose and that we cherish."

The revelations on Tuesday about the upcoming extremism bill came after the home office was forced to defend a counter-radicalisation programme by its research, information and communications unit (Ricu).

The scheme disguised the government's role in a number of propaganda campaigns aimed at young Muslim men, according to the Guardian.

Imran Khan, a human rights lawyer, told the paper: "If the government wants its Muslim citizens to listen to it, it needs to be trusted. And to be trusted, it needs to be honest. What is happening here is not honest, it's deeply deceptive.

"Furthermore, this government needs to stop thinking of young British Muslims as some sort of fifth column that it needs to deal with."