Theresa May's Conservative Cabinet colleague attacked her pre-election plans to allow the broadcasting watchdog to vet television programmes for extremist content before they air as a potential infringement on free speech, a leaked letter has revealed.

Sajid Javid described the Home Secretary's proposal to give Ofcom the power to approve programmes before they are broadcast in an effort to weed out extremist content as reducing the watchdog to the role of a censor.

The leaked letter to David Cameron, published by the Guardian, revealed Mr Javid's deep concerns that Mrs May's plans would amount to "a fundamental shift in the way UK broadcasting is regulated, away from the current framework which is designed to take appropriate account of the right to freedom of expression".

Mr Javid pointed out that other countries which have imposed similar powers "are not known for their compliance with rights related to freedom of expression and the Government may not wish to be associated with such regimes".

He sent the letter on March 12 when he was culture, media and sport secretary to inform the Prime Minister that he could not support Mrs May's counter extremism strategy and sent a copy to the Home Secretary.

The letter was written in response to a request from Mrs May for approval for the strategy from ministers in the Cabinet's home affairs committee and the national security committee.

The publication of the strategy to tackle non-violent extremism was delayed around the same time the letter was sent, with many observers believing the Liberal Democrats in coalition had blocked it.

Mr Cameron last week outlined plans to fast-track powers to tackle radicalisation, including a commitment to give Ofcom a strengthened role in taking action against channels which broadcast extremist content, alongside banning and disruption orders for people who seek to radicalise others or use hate speech in public.

It is not clear whether the Government has revisited Mrs May's plans since taking office, or whether they could be included in next week's Queen's Speech.

In the letter published by the Guardian, which is on Department for Culture, Media and Sport-headed paper, Mr Javid wrote: "Ofcom does not have the powers to approve programmes before they are broadcast and nor do we consider that it should have these powers as has been proposed in paragraph 111 of the strategy.

"Extending Ofcom's powers to enable it to take pre-emptive action would move it from its current position as a post-transmission regulator into the role of a censor.

"This would involve a fundamental shift in the way UK broadcasting is regulated, away from the current framework which is designed to take appropriate account of the right to freedom of expression.

"Whilst it is absolutely vital that Government works in partnership with individuals and organisations to do all it can to ensure that society is protected from extremism, it must also continue to protect the right to freedom of expression and ensure that these proposals do not restrict or prevent legitimate and lawful comment or debate.

"It should also be noted that other countries with a pre-transition regulatory regime are not known for their compliance with rights relating to freedom of expression and Government may not wish to be associated with such regimes."

Mr Javid, now Business Secretary, went on to express worries that the powers may be used in ways not intended by Mrs May because it is difficult to define what constitutes extremist content.

He also warned that the Government may be accused of "inappropriate involvement" in media regulation, an area that could be seen as sensitive following the Leveson Inquiry into the press following the phone hacking scandal.

Mr Javid wrote: "I am concerned about the risk that the powers would be used otherwise than intended, not least given the difficulty of defining extremism, and the consequent likelihood of the Government being seen to be interfering with freedom of speech without sufficient justification.

"Given that Ofcom comprises a Government appointed board there is clearly a risk that the Government will be accused of inappropriate involvement in the regulation of the broadcast media which is particularly sensitive in light of Leveson."

Mr Javid then said he could not agree the publication of the strategy if they contained Mrs May's plans for Ofcom and suggested they were replaced with a paragraph containing a commitment to keep the watchdog's powers in relation to extremism under regular review.

Both the Home Office and Downing Street said they would not comment on leaked correspondence.