Political activist and commentator Dr Shola Mos-Shogbamimu shared an image of the letter, signed by a group calling itself the National Action London Cell, which contained racist abuse and threats to her life and the lives of her family.

Presenter and transgender activist India Willoughby replied to Dr Mos-Shogbamimu's tweet to say she had also received a letter.

She wrote: "Dr Shola and I have both received hand-delivered letters from an emboldened British nazi group today. I know they attend Gender Critical rallies. State of this country."

Metropolitan Police said its Counter Terrorism Command was investigating due to the "potential involvement of a proscribed group".

National Action, which was founded in 2013, has been a proscribed organisation in the UK under the Terrorism Act 2000 since December 2016.

Since then, it has split into regional factions with bases around the country.

Police say they were first made aware of the letters as a result of Twitter posts on Monday.

Officers have contacted the victims to request statements and offer support.

Dr Mos-Shogbamimu, who regularly appears on TV to debate issues including diversity and women's rights, tweeted: "I have received 'a serious threat from National Action London' notifying me of their 'intention to kill' me & my family.

"They name my husband & home address. They don't consider killing me 'murder' they call it an 'execution'. This letter was through my front door.

"This is Britain."

Dr Mos-Shogbamimu shared an image of the letter, which contained vile racist abuse and death threats.

The commentator appeared on Sky News in January to discuss the killing of Tyre Nichols in the US.

'Threatening to kill me because I'm trans'

Ms Willoughby said she had received a similar letter delivered to her accountant.

"My accountant just called. He's received a letter today - hand delivered - no stamp. Threatening to kill me. Because I'm trans," she tweeted.

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"Full of graphic detail about what they are going to do. Passed on to the police.

"This is what Brit Media, the GC [gender critical] movement and the Gov have done."

She added: "Says it's like a letter from a horror film. He's obviously worried too, given how it was delivered."

Messages containing racist abuse

Dr Mos-Shogbamimu said she had also been receiving calls on her mobile phone that the caller ended as soon as she picked up, along with messages containing racist abuse.

Responding to her post, Labour MP Jess Phillips said: "All my love Shola, please shout if I can help."

No arrests have been made and enquiries continue.

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Last year the founder of National Action, Alex Davies, was found guilty of terrorism offences and sentenced to eight-and-a-half years in prison.

The "extremist's extremist" was the last of 25 members of the neo-Nazi group to be jailed.