The chairman of the Police Federation for England and Wales Paul McKeever has died after suffering from an embolism.
The 57-year-old, who was married with one daughter, announced in July last year that he would retire from his role this month.
He died on Thursday night after being admitted to hospital a few days ago.
Chair-elect of the group Steve Williams said he was "terribly saddened" by the news.
"He was a truly outstanding chairman, and most importantly a truly outstanding police officer and man. A true gentleman, his leadership and reputation will be remembered highly by all those who knew him," he said.
Metropolitan Police Federation chairman John Tully said: "It has come as a total shock to everyone, it is completely out of the blue.
"He led the Federation through perhaps the most demanding time in history and was within two weeks of retirement."
Mr McKeever, who was appointed chairman in 2008, was recently embroiled in the so-called 'plebgate' scandal involving former chief whip Andrew Mitchell.
Mr Mitchell was forced to quit his Cabinet post amid a storm of protest - fuelled by the Police Federation - over claims that he called officers "plebs" during a spat in Downing Street.
Earlier in the affair, Mr McKeever stepped up pressure on Mr Mitchell to hand in his resignation, saying it was "hard to fathom how someone who holds the police in such contempt could be allowed to hold a public office".
But subsequent claims suggested Mr Mitchell could have been the target of an alleged conspiracy involving officers, which is now the subject of a Scotland Yard investigation.
Mr McKeever acknowledged concerns that his organisation had "stoked up" attention on Mr Mitchell, adding that he would apologise to the MP if the new investigation showed he had been wrongly accused.
When announcing his retirement last year, Mr McKeever said: "I cannot stay within a service that is having the Office of Constable attacked, police officers denigrated and public safety put at risk.
"I will be able to fight freely outside the service; I may be retiring but I will not stop fighting for what is right and for public safety."