The Queen's cousin has told the BBC she is "perfectly sure" the monarch will never retire or abdicate the throne.
The Hon Margaret Rhodes was speaking as the Queen began her birthday celebrations with a lunch for 99 people who also turn 80 on 21 April.
She said: "It's not like a normal job, it's a job for life." The vows made on Coronation Day were "so deep and so special" to the Queen, she added.
"She wouldn't consider not continuing to fulfil those vows until she dies."
Ms Rhodes and the Queen have been close friends all their lives, and they meet regularly.
When asked by BBC royal correspondent Nicholas Witchell if she was categorically saying the Queen would neither retire nor abdicate, but would remain in the role until her death she said: "Yes, I'm perfectly sure that's what will happen."
Ms Rhodes also told the BBC the Queen had been increasingly enjoying her role as monarch in recent years.
"I think in a funny way, perhaps, the death of Queen Elizabeth the Queen Mother had quite a huge effect on the Queen.
"Not only of sadness, but in a way that she could come into her own as the head of the family and as the most senior royal lady," she said.
Meanwhile, Prime Minister Tony Blair also praised her in Parliament for her "profound impact on our country".
On Wednesday the Queen's guests - who were selected in a ballot earlier in the year - gathered at Buckingham Palace for a three-course meal in the grand Palace Ballroom.
They applied for the ballot through their local Lords Lieutenant throughout Britain, and each can bring one person with them.
Four people attending the lunch were from overseas - two from Canada, one from Australia and one from New Zealand.
The guests dined around 20 tables inside the royal home, with some seated next to the Queen or Duke of Edinburgh.
They were also invited to a drink in the palace's Picture Gallery before lunch.
In her welcoming speech, the Queen said: "I doubt whether any of us would say the last 80 years has been plain sailing, but we can give thanks for our health and happiness, the support we receive from our families and friends, some wonderful memories and the excitement that each new day brings."
Guests, wearing their best dresses and suits, told of their excitement as they arrived at the palace.
Doreen O'Leary, from Oswestry in Shropshire, said: "I feel wonderful, this is something I never dreamed of.
"I've known all my life that I've shared my birthday with the Queen but today I feel quite emotional."
Jemma Phipps picked the outfit. Pic: by kind permission of Ascot Racecourses Ltd
Also marking the Queen's birthday is the unveiling of a new portrait showing her dressed in pink and ready for the races at Ascot.
The painting, by 28-year-old Jemma Phipps, was commissioned by the Duke of Devonshire to celebrate both the Queen's birthday and the racecourse's revamp.
The Queen will spend her 80th birthday at Windsor Castle, where Prince Charles will host a family dinner.
Other events are planned including a service of thanksgiving at St George's Chapel, Windsor on 23 April, and a service at St Paul's Cathedral on 15 June.
The monarch will also host a birthday garden party at Buckingham Palace for children from all over the UK.
On 17 June, the Queen will mark her official birthday with the Trooping the Colour.
Republic, which campaigns for the monarchy to be scrapped, has used the birthday celebrations to renew its calls for a debate on the future of the Royal Family.