Counter-terrorism police in the UK are investigating a suspected far-right extremist group after a threatening mail and suspicious packages were sent to mosques across the country.

Three mosques in London are the latest to receive malicious letters from the unknown individual or group as part of what a monitoring group believes is a “structured” transatlantic campaign, ‘The Independent’ reported.

A letter seen by the newspaper threatened to bomb Forest Gate Mosque in London “very soon...when there are plenty of Muslims”, while the sender told another mosque in east London they would “make you vermin pay” and a third letter received in Hackney called for the capital to be “cleansed”.

Tell Mama (Measuring Anti-Muslim Attacks), which records Islamophobic hate crimes in Britain, said the letters appeared to be part of the same campaign that targeted three other mosques in the UK last year and others in the US.

Steve Rose, who has been documenting the phenomenon for the charity, said three mosques in Tottenham, Leyton and Finsbury Park in London received envelopes scrawled with abuse and containing white powder in July last year.

“What was interesting was the relative proximity of these mosques. It seems whoever it is targeting geographic areas of London and might have targeted a mosque in Rotherham as well,” he said.

The revelations came as it emerged that around 40 neo-Nazis are being investigated by UK police amid fears that they are plotting terrorist attacks against Muslims around the country.

The extremists are understood to be operating in “far-right hotspots” predominantly in Yorkshire, including Leeds, Dewsbury and Batley in northern parts of England, ‘The Sunday Times’ quoted police intelligence sources familiar with the investigations as saying.

“The danger far-right extremists pose to national security is no different from Islamist terrorists,” the source said.

The UK’s North East Counter Terrorism Unit, which covers Yorkshire and is understood to be receiving tactical support from MI5 intelligence service, said: “Over the past year or so, there have been indications the threat from extreme right-wing [individuals] could be increasing...UK counterterrorism policing is alive to this.”

The neo-Nazis being investigated are “proactively plotting” by familiarising themselves with Islamic institutions and community representatives.

This contrasts with those who carry out hate crimes against Muslims in response to Islamist terrorist attacks.