Amid the turmoil of militant attacks, political crisis and a deadly fire disaster, Queen Elizabeth said Britain was in a deeply sombre mood but that its people were resolute in the face of adversity.

Scarred by three deadly Islamist militant attacks and a tower block fire that killed dozens in London, Britain is also grappling with a political crisis that has left Prime Minister Theresa May weakened on the eve of Brexit talks.

"It is difficult to escape a very sombre national mood," Elizabeth, the world's oldest and longest-reigning monarch, said in a message on her official birthday. "The country has witnessed a succession of terrible tragedies."

Elizabeth said that during recent visits to meet victims she had been struck by the inclination of people to offer comfort and support to those in need.

"Put to the test, the United Kingdom has been resolute in the face of adversity," she said. "United in our sadness, we are equally determined, without fear or favour, to support all those rebuilding lives so horribly affected by injury and loss."

Elizabeth was born on April 21, 1926 in Bruton Street in central London when Calvin Coolidge was US President and Josef Stalin had just taken control in the Soviet Union. She became queen in 1952, aged 25.

At 91, she still regularly carries out official duties but has cut back on her number of engagements in recent years. She celebrates two birthdays each year: her actual birthday on 21 April and her official one which usually falls on the second Saturday in June