Police have warned nearly 2,000 people their lives were under threat in the last four years, according to figures. There have been at least 1,948 "threat to life" notices issued by 27 police forces in the UK since 2012.
But the actual number could be much higher as police in the West Midlands, Merseyside and Greater Manchester refused to provide figures. The National Police Chiefs' Council (NPCC) said the letters had proven "highly effective" in most cases.
The letters - also known as Osman warnings - are issued by the police if they become aware of a real and immediate threat to somebody's life.
Press Association figures showed 210 notices were issued by Police Scotland last year. It refused to provide information for 2015 over concerns it could identify the people involved.
Between 2012 and 2014, Nottinghamshire Police issued 248, while the Metropolitan Police issued 207. In the same period, South Wales Police issued 97 and Kent Police issued 117.
An NPCC spokesman said: "Osman warnings are one of a number of options that police forces can use to deal with a situation when there is a threat to an individual's life.
"The police response will always need to be proportionate and will be relevant to the unique factors of any specific threat situation under consideration, taking into account all available information including the level of the threat and how imminent it is.
"Osman warnings have proved highly effective in the overwhelming majority of cases." Some forces refused to reveal the number of notices they had issued
The letters were started after Ali Osman, a businessman, was shot dead in east London in 1988. Paul Paget-Lewis, the teacher who killed him, had previously said he was thinking of committing a massacre.
Paget-Lewis also shot Mr Osman's son after getting to know him at the school he worked at. He was convicted of manslaughter on the grounds of diminished responsibility.
Many forces did not reveal the numbers under Freedom of Information laws, saying it would cost too much. But other figures showed: Bedfordshire Police issued 82 threat to life notices to men and women aged between 17 and 74 from 2012 to August this year
The 41 notices issued between 2012 and 2014 by Durham Constabulary included a boy and a girl under the age of 18 in 2012 and a male aged over 60 in 2013
The Police Service of Northern Ireland (PSNI) did not disclose how many warnings were issued for the last three years but said 2,320 were sent out in 2010, compared with 890 in 2011 and 258 in 2012.