* Elizabeth passes 63-year milestone of Queen Victoria
* Celebrations will be low-key, but queen may make speech
* Queen will be in Scotland to open new railway line
By Michael Holden
EDINBURGH, Sept 9 (Reuters) - Queen Elizabeth, who ascended
the throne aged just 25 as her exhausted country struggled to
recover from the ravages of World War Two, makes history on
Wednesday when she becomes Britain's longest-reigning monarch.
At about 5.30 p.m. (1630 GMT), Elizabeth, who now aged 89 is
also the nation's oldest ever monarch, will surpass the 63
years, 7 months, 2 days, 16 hours and 23 minutes that her
great-great-grandmother Queen Victoria spent on the throne.
British Prime Minister David Cameron led tributes to the
queen.
"Over the last 63 years, Her Majesty has been a rock of
stability in a world of constant change and her selfless sense
of service and duty has earned admiration not only in Britain,
but right across the globe," Cameron said.
"It is only right that today we should celebrate her
extraordinary record, as well as the grace and dignity with
which she serves our country."
Those close to the queen say she is fairly blase about the
milestone, believing it represents little more than the fact she
has lived for a long time.
"It's business as usual as far as she is concerned," said
one senior aide.
Initially she did not even intend to mark the event
publicly, but she has bowed to public pressure and will now
undertake an official engagement in Scotland, where she
traditionally spends her Summer holiday.
Along with her husband Prince Philip, who has been at her
side throughout her reign, she will take a journey on a steam
locomotive to mark the opening the longest domestic railway to
be built in Britain for more than 100 years.
A Buckingham Palace spokeswoman said she might even make a
rare public speech afterwards.
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Take a Look - Background to the milestone ID:nQUEEN
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"It's inevitable that I should seem a rather remote figure
to many of you, a successor to the kings and queens of history,"
she said in her first televised Christmas broadcast in 1957.
"I cannot lead you into battle. I do not give you laws or
administer justice. But I can do something else. I can give you
my heart and my devotion to these old islands and to all the
peoples of our brotherhood of nations."
As a young princess, Elizabeth had not expected to become
monarch as her father George VI only took the crown when his
elder brother Edward VIII abdicated in 1936 to marry American
divorcee Wallis Simpson.
She was 25 when she ascended to the throne on Feb. 6, 1952,
following her father's death.
That made her the 40th monarch in a royal line that traces
its origin back to Norman King William the Conqueror who claimed
the throne in 1066 with victory over Anglo-Saxon Harold II at
the Battle of Hastings.
The year she became queen, the Korean War was raging, Joseph
Stalin was leader of the Soviet Union and Britain announced it
had the atom bomb.
"There is a motto which has been borne by many of my
ancestors - a noble motto, 'I serve,'" Elizabeth said in a
televised speech to mark her 21st birthday.
"I declare before you all that my whole life, whether it be
long or short, shall be devoted to your service and the service
of our great imperial family to which we all belong."
Since becoming queen, she has seen 12 prime ministers,
starting with Winston Churchill, and there have been 12 U.S.
Presidents, from Harry S. Truman to Barack Obama.
Not only has Elizabeth reigned the longest but according to
a poll in the Sunday Times this week, Britons also think she is
the country's greatest monarch, ahead of her Tudor namesake
Elizabeth I and Victoria, who was queen for much of the 19th
century when Britain built up its empire.
However, not everyone has been impressed, with republicans
saying her silence on political matters is her finest
achievement, and well-known British historian David Starkey
remarking that she had never said or done anything memorable.
"She will not give her name to her age. Or, I suspect, to
anything else," Starkey wrote in the Radio Times magazine.
(Editing by Stephen Addison)