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 West Midlands, Merseyside and Greater Manchester were among the forces that 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 police if they become aware of a real and immediate threat to somebody's life.
Police Scotland issued 210 last year, the Press Association figures showed. The force refused to provide information for 2015 over concerns those involved could be identified.
The Police Service of Northern Ireland did not disclose how many warnings were issued for the last three years but said 2,320 were sent in 2010, 890 in 2011 and 258 in 2012.
Many forces did not reveal the numbers, saying it would cost too much.
What is an "Osman warning"?
Police issue an "Osman warning" letter when there is intelligence of a threat to someone's life, but not enough evidence to justify the police arresting the possible offender.
They are named after London businessman Ali Osman, who was murdered by Paul Paget-Lewis, a teacher at his son's school, in 1988.
Courts heard Paget-Lewis had formed a "disturbing" attachment to Mr Osman's son, Ahmet, then 14, and at one point told police he was considering committing a massacre.
In March 1988, he stole a gun, killed Mr Osman and shot and seriously injured Ahmet.
In 1998, the Osman family successfully argued in the European Court of Human Rights that the Metropolitan Police had breached Mr Osman's right to life because it had all the information it needed to deal with the threat.
It was a significant ruling which lead to police developing a complex methodology for assessing risks, and the formal "Osman warning" letters were introduced.
Len Gridley, an opponent of the Dale Farm travellers' site - which was at the centre of a long-running occupancy dispute until families were evicted in 2011 - said he received a notice after a death threat was made against him.
"The police sent me a letter and sent someone over to check the safety of my property," he said.
"When you receive something like that, it puts more things in your mind. You're a bit more aware and safety conscious.
"As time passes, you can't let it rule your life, otherwise they win. It was a waste of time in my case but I'm not saying that applies to other people though."
In 2011, Simone Banarjee described her reaction when police arrived on her doorstep with an Osman letter warning that her fiance Malcolm Webster was believed to be a murderer.
The 52-year-old was subsequently found guilty of killing his former wife Claire in Aberdeenshire in 1994, and of trying to kill his second wife five years later.
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."
Many forces did not reveal the numbers under Freedom of Information laws - 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
Have you received one of these letters? You can share your comments and experience in confidence by emailing firstname.lastname@example.org.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: