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ISIS fanatics used the secretive messaging site Telegram to call for a “lone wolf” attack on Parliament just weeks before Khalid Masood struck.

A Sunday Mirror probe has uncovered chilling messages in which jihadi masterminds urged terrorists to mount atrocities in the UK.

They shared an image of an IS fighter dressed like executioner Jihadi John, wielding a sword in front of Big Ben.

The illustration – headlined Fight Them – showed a fireball and a tattered Union Flag flying from a pole.

Just weeks later Masood, 52, ploughed his car into scores of innocent bystanders on Westminster Bridge, killing three and injuring 50.

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Then he rammed gates at Parliament and stabbed married dad PC Keith Palmer, 48, to death – before being shot dead himself.

Masood had been sending messages via WhatsApp in the moments before Wednesday’s attack.

The Met Police revealed investigations suggest he acted alone.

But details of the online exchanges will mount fresh pressure on Telegram bosses to stop extremists from using it as a platform to recruit.

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The social network site – formed by Pavel Durov, 32, in 2014 before he fled his native Russia over fears for his life – is a go-to tool for terrorists because it commands “end-to-end encryption”.

It means security forces are unable to hack the service, which has more than 100 million active users, or intercept the information they share.

Durov claims there is “little you can do” to stop terrorists using the service.

Extremists used it to exchange messages after the Berlin Christmas market atrocity and the Nice terror attack.

(Image: Getty Images Europe)

Messages uncovered during our probe show political leaders, Jewish schools, museums, pubs and clubs were identified as “perfect targets”.

One message, showed to the Sunday Mirror by a source, listed a series of recent strikes mounted by IS, before asking: “So what’s next? London? Berlin? Moscow? Add your city here.”

It then answered the question with: “Britain.”

Another message we saw said: “To attack any individual or a ­nation’s pride and joy is to cause an utmost amount of hurt/pain to the enemies of Allah who are attached to this dunya [earthly world].”

(Image: AFP)

Jihadists urged targeting of football matches – similar to December’s Istanbul attack which killed 38 – and gave advice on where security “may be more relaxed”.

The fanatics said: “Methods can include entering the stadium and detonating an explosive(s).

“Attacking fans/security at full time in the vicinity of the car park area or exits of the stadium.

"Devices can be left in around the stadium, bars, cars, busses, trains, transportation etc.

(Image: Getty Images Europe)

“Attacks can compromise of explosives, gun attacks, knife, martyrdom vests, CHEMICAL and any other.”

One group launched guides for terrorists six weeks ago – on February 13.

Hundreds of videos, bomb-making guides and military manuals were also uploaded to the site.

A league table of deaths caused by IS attacks was posted – issuing a challenge to fanatics to top the 130 fatalaties in the 2015 Paris attacks.

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Videos showed news coverage of attacks, including the murder of soldier Lee Rigby in Woolwich and the 7/7 London bombings.

One message written in English said: “Footage of some of the previous operations of the past to inspire you!”

Police probing the Westminster attack are still holding a 58-year-old man arrested in Birmingham.

Two others, picked up in Birmingham and East London, were released while a woman, 32, arrested in Manchester is on bail.

Spotlight on Telegram

Telegram has faced criticism for failing to stop jihadists using it to peddle extremism.

Islamic State’s previous recruiting tool was Twitter – which this week claimed to have closed 376,890 terror-linked accounts.

But Telegram’s encryption means messages are visible only to the person who has sent them and the individuals meant to receive them.

A “lock” prevents hackers and “oppressive regimes” spying.

Jihadist Rachid Kassim, who was killed in a drone strike last month, used Telegram to plot dozens of attacks in his native France.

Telegram also allows IS to distribute terror manuals and their official magazine.