The Queen, who celebrated her 94th birthday on 21 April, is the longest-reigning monarch in history with 67 years under her belt.

However, before assuming her regal responsibilities in 1952, the Queen took on another role, joining the Women’s Auxiliary Territorial Service during the Second World War.​

The monarch’s position as a driver and mechanic during the war is being explored in a documentary airing at 9pm on Wednesday 22 April on ITV, titled Our Queen at War.

Here is everything you need to know about Princess Elizabeth’s past in the armed forces.

When the Second World War began in 1939, Princess Elizabeth was 13 years old while her younger sister, Princess Margaret, was nine.

It was suggested that the young princesses should be evacuated out of the country to North America or Canada.

However, the Queen Mother (who was Queen at the time) was adamant in her refusal to leave, stating: “The children won’t go without me. I won’t leave the King. And the King will never leave.”

The princesses thus remained in the UK, flitting between Balmoral Castle in Scotland, Sandringham House and Windsor Castle, where they eventually settled for several years.

In 1940, Princess Elizabeth made her first radio broadcast, addressing child evacuees.

“We are trying to do all we can to help our gallant sailors, soldiers and airmen, and we are trying, too, to bear our own share of the danger and sadness of war,” she said during BBC’s Children’s Hour.

“We know, everyone of us, that in the end all will be well; for God will care for us and give us victory and peace. And when peace comes, remember it will be for us, the children of today, to make the world of tomorrow a better and happier place.”

As Princess Elizabeth neared her 18th birthday, it became apparent that she felt keen to help in the war effort.

In 1945, Life magazine published an article that stated that after “long deliberations”, King George VI had decided that “her training as a princess outweighed the nation’s increasing manpower problems and that ‘Betts’ should not join any of the women’s auxiliaries, nor work in a factory”.

Nonetheless, shortly thereafter, it was declared that the heir presumptive had been granted a commission as an honorary second subaltern in the Auxiliary Territorial Service (ATS) on 24 February 1945.

The announcement was made in a supplement to the London Gazette on 9 March 1945.

Once she joined the Auxiliary Territorial Service, Princess Elizabeth trained as a driver.

After passing the military driving test, she became a driver for the Second Subaltern Windsor Unit.

The princess also learnt how to repair vehicle engines as a mechanic in her unit.

She was later promoted to the rank of honorary junior commander, which at the time would have been the female equivalent of becoming a captain.

One day, she received a visit from her mother, Queen Elizabeth, who watched as her daughter explained what her role entailed.

In the issue of Life magazine published in 1945, it explained that while the princess took up service in the armed forces, she “did not sleep at the camp but motored each night to Windsor Castle, showing up the next morning at 9 o’clock”.

The teenager was reportedly surprised by the amount of work that went into preparing for a royal engagement when she was visited by her parents, the King and Queen.

“I never knew there was quite so much advance preparation. I’ll know another time,” she said as she helped to clean the camp.

“It seems so strange to watch them drive away,” the princess added.

The Queen became the first female member of the royal family to serve in the armed forces and is the only living head of state to have served in the Second World War.