The Queen will carry out light duties as she continues to rest following her overnight stay in hospital for “preliminary investigations”, it is understood.

The 95-year-old monarch, who was ordered to rest by doctors and advised to miss a trip to Northern Ireland this week, is staying at her Windsor Castle home and is said to remain in “good spirits”.

Prime Minister Boris Johnson has sent his “best wishes” to the head of state, who a source said would be “resting and undertaking light duties”.

Her short period at King Edward VII’s hospital in London was kept a secret by Buckingham Palace, which has left some media outlets frustrated and citing issues of trust.

It is understood the Queen was due to stay for only a short period while seen by specialists, so the development was not announced by the Palace at the time, and protecting her medical privacy was also a consideration.

The overnight admittance was for “practical reasons”, a source said.

The Queen’s trip by car rather than helicopter to the central London private hospital was confirmed by the Palace on Thursday evening only after The Sun newspaper broke the news.

A Buckingham Palace spokesman said late on Thursday night: “Following medical advice to rest for a few days, the Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today, and remains in good spirits.”

Joe Little, managing editor of Majesty Magazine, said the issue of revealing details about the monarch’s health is problematic: “This is a tricky one because the Queen does have a right to a certain degree of privacy, but on the other hand she’s head of state.

“So does that entitle us to know exactly what ailments she may or may not have? It’s a very difficult one to get the balance right for the satisfaction of everybody.”

The Queen’s admittance was her first overnight stay in hospital since she spent a night at the private clinic in 2013 when she was treated for the symptoms of gastroenteritis.

A statement released by Buckingham Palace on Wednesday at 11am, before the Queen was admitted to hospital that afternoon, announced the cancellation of her two-day visit to Northern Ireland.

It said “The Queen has reluctantly accepted medical advice to rest for the next few days”, and went to say she was “disappointed” at not being able to travel and sent her “warmest good wishes” to the people of Northern Ireland.

Royal author Penny Junor said the Palace’s decision not to announce the hospital visit prevented unfounded rumours circulating.

She said: “I’m not surprised they did that, otherwise there would be speculation. People would be putting her in her grave long before she’s ready for it.”

A Downing Street spokesman declined to say whether Mr Johnson had held his weekly audience with the Queen, but so far it has not been listed in the Court Circular, the official record of royal engagements.

The spokesman said: “The Prime Minister’s best wishes have been passed on to Her Majesty.”