Queen Elizabeth II is resting at Windsor Castle after "reluctantly" accepting medical advice to cancel a trip to Northern Ireland, planned for Wednesday and Thursday, Buckingham Palace has said.

The 95-year-old UK monarch has been advised to rest for the "next few days," the palace said in a statement.

A source close to the palace told CNN that it is "not Covid related."

"Her Majesty is in good spirits and is disappointed that she will no longer be able to visit Northern Ireland, where she had been due to undertake a series of engagements today and tomorrow," the statement said.

"The Queen sends her warmest good wishes to the people of Northern Ireland, and looks forward to visiting in the future," it added.

The Queen, along with British Prime Minister Boris Johnson, had been due to attend a church service in the city of Armagh on Thursday, marking the centenary of partition and Northern Ireland's foundation.

Irish president Michael D. Higgins had earlier declined an invitation to attend the event.

The Queen's medical update comes days after she turned down a magazine's attempt to award her the title of "Oldie of the Year," saying she felt too young at heart.

She is the world's longest-reigning monarch, presiding over the nation for almost seven decades.

She became Queen in 1952, aged 25.