A schizophrenic man who sent an offensive letter claiming to contain anthrax to the prime minister has been jailed.

Vincent Potter, 60, was given an 18-month prison sentence at the Old Bailey.

Judge Rebecca Poulet QC rejected the defence's call to punish him with a community order but said she cut the jail term because of his mental illness.

“In my judgment the person sending this frightening letter to the prime minister intending them to fear for their safety must be punished by immediate custody," she told the court,

Potter's letter, containing a substance likened to “biscuit crumbs”, was intercepted before it reached Theresa May and sparked a counterterror investigation.

“Dear bitch,” it began, before claiming that the envelope contained a “little surprise for your officers”.

“The powder in this envelope is anthrax,” the letter claimed. "This comes from the Brothers of the Right Hand.”

Potter, who suffers from schizophrenia, had written the letter on a typewriter and addressed it to the prime minister at the Houses of Parliament.

The Metropolitan Police said it aroused suspicion at a London postal sorting office. 

Employees wearing protective clothing found that it “contained a white powder along with a note indicating that the powder was anthrax”.

The letter was addressed to Theresa May at the Houses of Parliament  (Getty)

“The letter was seized by police and the powder was assessed and determined not to be noxious,” a spokesperson for Scotland Yard said. 

“Further forensic work on the envelope and letter found traces of DNA, which led officers to identify Potter. 

“He was arrested by officers from the Met’s Counter Terrorism Command at his home address in October 2017 and questioned by detectives.”

Potter’s DNA was found on the stamp and a typewriter thought to be the one used to write the letter was discovered at his home, in the village of Mayfield in East Sussex.

Prosecutors said there was no evidence of any specific terrorist motivation by the letter's author, who entered the Old Bailey courtroom with a walking stick to be sentenced.

He previously pleaded guilty to making a noxious substance hoax on or before 23 August last year, under section 114(2) of the Anti-Terrorism, Crime and Security Act 2001.

The Old Bailey heard he may have been “in the grips of delusional episodes” when the letter was sent and had not been taking his medication for schizophrenia.

When questioned, Potter told police he had never heard of anthrax, or the “Brothers of the Right Hand” and said the letter was “nasty, unpleasant and wicked”.

Potter was also convicted 13 years ago for making an explosive over a dispute with a neighbour.

Judge Poulet acknowledged that he has suffered for many years with schizophrenia but said he could still present a danger to the public.

“I find [the explosive conviction] a very troubling aspect of this case and it is because of that background that I find myself unable to say that you do not present a risk or danger to the public,” she told the court.

The judge said a neighbour and friend of Potter's had said in a letter of support that he had described the offence as a "moment of madness".

A defence lawyer said Potter, who has 14 previous convictions for 25 offences including criminal damage and burglary, still has "no real recollection of the events".

Paula Bignall told the court her client, who was diagnosed with schizophrenia when he was 19, had not been taking his medication at the time of the incident and described his state of mind as having "degenerated quite significantly" in the months leading up to it.

He had no political affiliation and said he did not know why Ms May became the target of the letter, Ms Bignall added.

She described him as being "filled with remorse, regret and to some extent a lack of understanding and comprehension as to how he committed the offence".

"He still has no real recollection of the events," she said. "At the time it seems he may well have been in the grips of delusional episodes."

Additional reporting by PA