Britain has celebrated Queen Elizabeth II becoming the country’s longest-serving monarch with a flotilla down the River Thames, a gun salute and the peal of Westminster Abbey’s bells.

The queen herself opened a railway line in Scotland on Wednesday and was to host a private dinner at Balmoral Castle to mark the day she overtakes her great-great grandmother queen Victoria’s record.

A cheering crowd and an honour guard of royal archers greeted Elizabeth II on her arrival at Edinburgh train station, where she boarded a steam train for a journey along the new Borders Railway.

In pictures: the reign of Queen Elizabeth II

Dressed in a turquoise coat and hat and clutching a black handbag – one of her famously colourful outfits – the queen wore a diamond-studded brooch that belonged to Victoria in homage to her ancestor.

Prime Minister David Cameron led official tributes in parliament, calling her reign a “golden thread running through three post-war generations.

“She is our queen and we could not be more proud of her,” he said. “She has served this country with unerring grace, dignity and decency and long may she continue to do so.”

The Royal Barge Gloriana leads a procession along the River Thames. Photo: Getty

It is not known where exactly she will be at 1630 GMT (0230 AEST Thursday), the best estimate from royal officials for the time at which the monarch, who has become synonymous with Britain itself, reaches the landmark.

At that moment, the 89-year-old monarch will have served 23,226 days, 16 hours and roughly 30 minutes on the throne – over 63 years.

The queen is expected to comment on the occasion later on Wednesday, although a royal source has said she wants to keep the occasion low-key.

“While she acknowledges it as an historic moment, it’s also for her not a moment she would personally celebrate, which is why she has been keen to convey business as usual and no fuss,” the source explained.

Queen Elizabeth II opens the Borders Railway in Scotland. Photo: Getty

The official photograph, taken by Mary McCartney, daughter of Beatles star Paul, shows the queen sitting at her desk working through a red box of state papers.

Elizabeth became queen upon the death of her father George VI, Britain’s king during World War II, whose youthful stutter inspired the Oscar-winning film “The King’s Speech”.

The calculation of her time on the throne is based on when he passed away, which is estimated at around 1am on February 6, 1952 – an hour after he was seen for the last time at his bedroom window at Sandringham House in eastern England.

The queen presided over a gradual decline in Britain’s global influence as many of its former colonies became independent, as well as a sharp rise in living standards and the advent of the digital age.

She has also steered the monarchy through some of its rockiest recent patches, including the collapse of three of her children’s marriages and public anger at her reaction to the death of Princess Diana in 1997, which some saw as cold.

The royal family has since tried to present itself as more in touch with the public.

That decision was crowned by the highly-popular marriage of the queen’s grandson Prince William to commoner Kate Middleton in 2011, and the birth of the couple’s two children, Prince George and Princess Charlotte.

A drummer in the pipe band plays as crowds wait to catcha glimpse of Queen Elizabeth II at Waverley Station. Photo: Getty