Portrait of the Queen unveiled by Boris Johnson at City Hall

It is nearly seven years since the Queen officially opened the new home of the Greater London Authority – which comprises the Mayor and the London Assembly – alongside the Thames near Tower Bridge.

"On moving into City Hall I was surprised that there was no portrait of the British head of state to be seen anywhere in the building," said Boris Johnson.

"The Queen should be recognised in all significant civic headquarters and I am delighted that this has now been put right at the GLA, and that we have been able to use it to mark the Queen's birthday."

"Through many of modern Britain's turbulent times the Queen has stood as an example of British resolve, fortitude, and good-spirit. These are qualities that I'm sure everyone working at and visiting City Hall can identify with and draw inspiration from."

The picture will also enable City Hall to host citizenship ceremonies during which new citizens will swear allegiance to the Queen in front of the portrait.

The picture, measuring 31 inches by 26 inches, has been installed near the lifts in the staff entrance foyer.

"Suspense built as the mayor made several attempts to draw back the curtain covering the picture," recounts Tory London Assembly member Roger Evans. "A good tug pulled it clear and Boris made a short speech whilst photographs were taken."

• A gun salute to mark the Queen's birthday will be fired from the Tower of London at 1pm on Tuesday 21 April.