Britain's biggest police force has set up a controversial unit – dubbed as ‘thought police’ by critics last night – to investigate offensive comments from the internet.

It will be supported by an army of volunteers trained to seek out anything they deem inappropriate on social networks such as Facebook and Twitter.

They will then report it to officers who will attempt to track down the culprits and possibly prosecute them, according to a report seen by The Mail on Sunday.

Scotland Yard is spending £1.7 million to set up its Twitter squad, which will have five detectives running it.

The Metropolitan Police, pictured, are spending £1.7million on a social media squad to investigate abusive comments and track down trolls

The establishment of the new unit comes after a surge in reports of racist and sexist abuse on social media, with some trolls jailed for making death threats against MPs.

But there have also been high- profile cases where police have been accused of being too heavy-handed in arresting or prosecuting people simply for making jokes.

Last night, MPs and civil liberties campaigners raised fears that the new unit would stop people expressing opinions for fear of arrest.

Liberal Democrat leader Tim Farron said: ‘We want more police on the street, not thought police.

‘Online bullying is an increasingly serious problem but police should not be proactively seeking cases like these and turning themselves into chatroom moderators.

‘With such measures, even if well intentioned, there is a real danger of undermining our very precious freedom of speech.’

Andrew Allison, of The Freedom Association libertarian group, said: ‘There’s a risk of online vigilantism, where people who are offended by the least thing will have a licence to report it to the police.’

And Frank Furedi, emeritus professor of sociology at the University of Kent, said: ‘Police are becoming moral arbiters rather than dealing with real issues that threaten our security.’

From the sinister...Bristol MP Thangam Debbonaire, pictured, complained about a tweet after a 20-year-old student said she should ‘get in the sea’ – a dismissive phrase regularly used on Twitter. The politician replied: ‘I believe that is a threat to kill.’

...to the ridiculous: After a relative posted this photograph on Facebook of the Siddiqui family paintballing – along with the tongue-in-cheek caption ‘ISIS training day’ – the family was visited by officers from Derbyshire Police. The fun-loving family regularly appear on the reality television show Gogglebox.

Documents seen by this newspaper show the Home Office is pouring £452,756 into the Online Hate Crime Hub, which is due to run for two years.

It will be headed by a detective inspector, a detective sergeant and three detective constables.

The London Mayor’s City Hall headquarters has advertised for a civilian programme manager who will be paid up to £52,455 – twice what a PC earns – to co-ordinate the project.

The detectives’ role will be ‘identifying the location of the crime’ when online abuse is reported, and refer it to ‘the appropriate force area and social media providers’.

Social media giants such as Twitter and Facebook – which do not always comply with police requests to obtain users’ details – will be asked to help fund a ‘community’ element to the unit, in which volunteers ‘skilled in the use of social media’ will ‘identify, report and challenge online hate material’.

It is feared that this will lead to large numbers of comments being reported to social media providers or police as inappropriate, even if they were only meant jokingly or had no malicious intent.

Robert Sharp, of the anti-censorship group English PEN, said: ‘Threats of violence must of course be investigated and prosecuted, but the police need to tread carefully.’

Lib Dem leader Tim Farron compared the unit to the 'though police' of George Orwell's dystopian classic 1984, pictured

London Mayor Sadiq Khan's (pictured) office is in favour of the unit because it wants to more to be done to bring hate crime offenders to justice

In the most notorious case of police over-reaction to a single tweet, Paul Chambers was fined for joking he would blow up an airport if it was closed by snowfall.

It took several years before his conviction was quashed on appeal.

John Cooper, the QC who defended Mr Chambers, said: ‘It does concern me that an aspect of policing is being sub-contracted to members of the public.’

Mother-of-two Debra Burt was questioned by police after writing on a friend’s Facebook page that she wanted to throw an egg at David Cameron.

Derbyshire Police visited the home of the Muslim Siddiqui family who appear on the television show Gogglebox after a relative posted a Facebook photograph of them paintballing with the jokey caption: ‘ISIS training day.’

Yesterday it emerged that a Labour MP had reported a student to the authorities at Bristol University for what she considered a death threat.

Verity Phillips, 20, tweeted to local MP Thangam Debbonaire that she should ‘get in the sea’ – a regular dismissive phrase used on Twitter.

The politician replied: ‘This person has just told me to drown – I believe that is a threat to kill.’

The London Mayor’s office is backing the new project as it believes ‘social media provides hate crime perpetrators with a veil of anonymity, making it harder to bring them to justice’.

It says almost half of hate crime against Muslims has taken place online, while almost one in five anti-Semitic incidents took place on social media.

There was a 42 per cent surge in reports of hate crimes – on the streets as well as online – in the wake of the EU referendum.

Under laws amended in 2015 amid growing concern at soaring abuse on the internet, trolls can now be jailed for up to two years for ‘malicious communication’.