"If anyone believes they have been a victim of such an offence we would encourage them to report it to police so it can be fully investigated".
The Metropolitan Police has confirmed it is aware of the leaflets being distributed in the capital.
In addition, they encourage people to "torture" and "butcher" Muslims in a day of Islamophobic violence on April 3. "We have told them to keep calm, and to phone the police if they receive one of these letters".
Several Britons in London, the West Midlands and Yorkshire said they received the printed letter through the post, according to reports, Al Jazeera news network reported.
Tell Mama, an organization that tracks anti-Muslim crimes, said in a Twitter post that the hate letters sent out in Britain numbered in the "double figures" and that it was working with the police to investigate the "malicious campaign".
Tell MAMA quoted the assistant chief constable of the South Yorkshire Police, Tim Forber, as saying the threatening letters were being investigated and details would be forwarded to the counter-terrorism police.
He said the letter had his name and business address on it, targeting him and potentially other recipients directly, rather than just sending them out at random.
"If any constituent has received a letter, or is concerned, contact me at email@example.com or if you'd rather, contact @TellMamaUK".
The letters also attempt to goad readers into carrying out attacks by questioning whether they are 'sheep.' "Sheep follow orders and are easily led, they are allowing the white majority nations of Europe and North America to become overrun by those who would like nothing more than to do us harm and turn our democracies in to Sharia led police states", the letter claims. It has a second class stamp on the plain white envelope and the frank mark is Sheffield.
"I can assure you that these documents are being taken extremely seriously and a thorough investigation into the circumstances is under way".
"Public safety remains our priority and I would urge our communities to be vigilant but not frightened".
Posters advertising a proposed "Punish a Muslim Day" have been sent to addresses in West Yorkshire.
West Yorkshire Police said it had received six reports while South Yorkshire Police said it had received three.
"Hate crime in any form will not be tolerated and we will work with our communities and alongside our colleagues in the counter-terrorism unit to ensure that those spreading fear and hatred will be brought to justice".
Tell MAMA [Measuring Anti-Muslim Attacks], a project that monitors Islamaphobic hate crimes in the United Kingdom, said a photo of an envelope suggested at least one of the notes was dealt with at a Sheffield sorting office.