Ross Kemp left unable to speak after trying Spice drug for Belmarsh documentary

Ross Kemp is exploring the notorious Belmarsh in his latest documentary. (TwoFour/ITV)

Ross Kemp has been inside one of the country's most notorious prisons for his new documentary but he was left unable to speak during filming when he tried Spice, a drug which has become alarmingly prevalent in UK jails.

The former EastEnders actor is seen smoking remnants of the drug via a prisoner’s vape pen in the series which sees him go into HMP Belmarsh in South East London.

Kemp wrote in The Sun: "I am rarely lost for words but as I turn to camera, I have lost the ability to speak. My brain has gone blank, my hands are clammy and my mouth is dry.

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"I have just inhaled a tiny amount of Spice residue left in a vape by a comatose prisoner. This particular version of the drug is known as Man Down — and it does exactly what it says on the tin."

Ross Kemp went into the Category A prison over a six month period. (Twofour/ITV)

The drug which is typically smoked by users is sometimes called "synthetic marijuana", however, its effects can be very different from cannabis.

It is made from dried plant matter and herbs that are sprayed with active ingredients and has the potential to be lethal, with it being blamed for a three-fold increase in "non-natural" deaths among prisoners.

Read more: What is ‘Spice’?

It has previously been blamed for rising violence in prisons after being smuggled in behind bars. Last year, it was reported that inmates at Leeds prison had even smoked dressing gowns soaked in the psychoactive substance to get high.

Kemp spent six months going inside Category A prison Belmarsh for the new two-part documentary for ITV.

Ross Kemp (right) meets Muhammad Asif Hafeez (left), known as Sultan, the alleged mastermind of a drug-smuggling empire. (Twofour/ITV)

The programme will see Ross interview Muhammad Asif Hafeez, nicknamed The Sultan, who was accused of being the mastermind behind a drug-smuggling ring.

Hafeez, who denies involvement in the crimes, is one of the prison's highest escape risks as he faces extradition to the US.

The prison has also been home to some of the country's most notorious criminals, including Ian Huntley, Lee Rigby's killers, Ronnie Biggs and black cab rapist John Worboys.

Welcome to HMP Belmarsh, With Ross Kemp starts Thursday 9 January on ITV.