The head of the Police Federation of England and Wales, Paul McKeever, has died after suffering an embolism.
Federation chairman Mr McKeever died on Thursday night after being admitted to hospital a few days ago.
Chair-elect of the group Steve Williams said: "I am terribly saddened to announce that Paul McKeever, chairman of the Police Federation of England and Wales, died last night. He was admitted to hospital a few days ago and sadly collapsed with a suspected embolism. We await further details. However, my deepest sympathy and that of the entire Police Federation is with his wife and family at this time.
"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."
Metropolitan Police Federation chairman John Tully said: "It is with profound sadness that we have today learned of the sudden death of our colleague and National Federation chairman Paul McKeever. Paul was a serving Metropolitan officer and was on the verge of retiring from the service after more than 30 years. Sincere condolences go to Paul's family, colleagues and friends at this very sad time."
Mr McKeever announced in July last year that he would retire from his role this month. At the time he 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."
Mr McKeever 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. Mr McKeever, who was 57, was married with one daughter.
Association of Chief Police Officers president Sir Hugh Orde said: "The sad news of Paul McKeever's death has come as a shock to the policing family. Paul and I served together as Pcs in London. Paul worked tirelessly in his role as Police Federation chairman and was a passionate advocate and voice for public safety and the rank and file. Our deepest sympathies and thoughts are with his family, friends and federation colleagues at this time."
Home Secretary Theresa May said: "I was deeply saddened to hear the news today of Paul McKeever's death. He gave more than 35 years of his life to the police in a long and distinguished career, including risking his own personal safety at the front line in the Brixton riots. As chairman of the Police Federation, he worked tirelessly on behalf of rank and file officers across the country and I know they will join with me in mourning his loss today."
Mrs May had clashed with Mr McKeever over police budget cuts and reforms to the service, such as lower starting salaries for new officers and proposals to introduce compulsory redundancy. At the last Police Federation conference in Bournemouth, he claimed the Home Secretary was "on the precipice of destroying a police service that is admired and replicated throughout the world".