London: Queen Elizabeth II became Britain's longest-reigning monarch after passing the record set by her great-great-grandmother Queen Victoria, the media reported on Wednesday.
The Queen, who is 89, will have reigned for 63 years and seven months - calculated at 23,226 days, 16 hours and approximately 30 minutes at about 17.30 (local time), BBC news reported.
Prime Minister David Cameron will lead tributes in the House of Commons and there will be a River Thames salute.
The exact moment the Queen reaches the milestone is not known because her father, George VI, passed away in the early hours of February 6, 1952.
On the Thames, a flotilla of historic vessels, leisure cruisers and passenger boats will take part in a procession between Tower Bridge and the Houses of Parliament.
The bridge will lift as a sign of respect and H.M.S Belfast will sound a four-gun salute.
The BT Tower in central London will scroll the message "long may she reign".
She has let it be known that she does not want a fuss to be made. It is evidently viewed as bad form for one long-lived queen to be celebrating the passing of a record set by another long-lived queen.
The prime minister's official spokeswoman said David Cameron had paid tribute to the Queen at a cabinet meeting on Monday.
He said the Queen had a "remarkable record" and was "a symbol of Britain's enduring spirit admired around the world".
Queen Elizabeth's reign has included 12 prime ministers, two more than served under Victoria.