Some of the letters have featured similar cartoons
The prime minister was among dozens of people across the UK who were targeted by a poison pen letter writer.
Police are trying to find who is responsible for more than 50 racially and sexually abusive letters.
Gordon Brown's Fife constituency office received a letter in April, while others were sent to mosques, hospitals, universities and private homes.
Much of the hate mail, mostly posted from Hampshire, includes references to "repatriation" and "exit Europe".
The first letter was sent in January 2007 with the most recent sent to a private address in Havant, Hampshire, in July 2009.
They have also been received in Derbyshire, Merseyside, Lincolnshire and Bedfordshire.
These forces had initially been conducting separate investigations until Hampshire Police set up Operation Heron in January 2009 to co-ordinate the search.
One of the letters was sent to Somers Park Primary School in Portsmouth.
Head teacher Janet Fleming said: "The content of the letter was sickening.
"But I would want to assure parents that this is a perfectly safe environment for their children to be in and that all the steps necessary to keep children safe were in place and still are in place.
The police say the campaign has a wide range of targets
"I am concerned that the person who sent it is not mentally stable. The language is appalling."
Ms Fleming believes the school was targeted because of the ethnic mix of its pupils where English is the second language for 26% of children.
Police said similarities in handwriting and content suggested the letters were the work of one person, with most of the mail postmarked Portsmouth and the Isle of Wight.
Det Insp Donna Goff, of Hampshire Constabulary, said: "Many of the letters people have found extremely distressing.
"Many people have felt extremely victimised by the letters."
The letters do not target a particular race, culture or religious group but are all pro-English, investigators said.
The inquiry will feature on the BBC1's Crimewatch programme on Tuesday night when officers will appeal for assistance in tracing the culprit.