Fourth plinth is reserved for the Queen

The fourth plinth in Trafalgar Square is being kept free for a statue of the Queen riding a horse which will be commissioned after she dies, say senior officials.

The plan sheds new light on why the plinth has never had a full-time occupant and has been used recently to showcase the work of modern artists. It also explains why the Mayor of London, who has been informed of the plan, recently performed a mysterious U-turn on proposals for a permanent statue to be placed on the monument, blaming "complex planning issues".

Even the Fourth Plinth Commissioning Group, which oversees what goes on to the famous space in the central London square, is believed to have been kept in the dark over the decision.

Between 1841 and 1999 there was nothing on the fourth plinth, which was sometimes referred to as the "empty plinth", before a string of modern designs adorned the space.

"It's perfect," a source said of the rotation system. "The modern art world doesn't want a permanent statue up there, and nor does the establishment."

Although the Department for Culture would not comment last night, four well-placed sources confirmed the long-standing decision to The Independent.

No single person took the decision, and those involved would not discuss it publicly, but in recent years the desire for a monument to the Queen has been the subject of discussion between No 10, the Palace and the local authorities.

One person from City Hall familiar with the discussions said the plan was for Her Majesty to be depicted riding. "The plinth is wide enough and perfectly shaped for Her Majesty on horseback," the source said.

The revelation explains why theformer mayor of London, Ken Livingstone, and his successor, Boris Johnson, would not commit to a permanent monument on the plinth in the north-west corner of Trafalgar Square by Nelson's Column.

In May, Mr Johnson withdrew his previous support for a permanent statue honouring the Battle of Britain hero Sir Keith Park. Before he was elected, Mr Johnson said he supported a campaign by Terry Smith, the chief executive of a City brokerage, to honour Mr Park, a New Zealand-born RAF commander seen as an unsung Second World War hero.

Later, in a written answer to a question from Jennette Arnold, a Labour member of the London Assembly, Mr Johnson supported the arrangement by which the plinth is reserved for contemporary sculptures which only stay in place for a year or two. Mr Johnson, who by this time had been told of the plan to honour the Queen, wrote of Mr Park: "The fourth plinth in Trafalgar Square would be a wonderful spot, but it appears complex planning issues would make it difficult to secure this location on an ongoing basis."

He went on: "There are also outstanding commitments to exhibit contemporary sculpture on the fourth plinth. I recognise that this revolving programme has proved very popular with the public and I welcome the important contribution it has made in shaping public debate about contemporary art. I am therefore exploring, with the Keith Park campaign, what is the best option available for them in view of these circumstances."

"It's as if Boris was told the nuclear secret," a source said.

Technically, the Greater London Authority is responsible for what is placed on the plinth, and Westminster City Council is in charge of granting planning permission for any proposed designs.

Last night, the chairman of the Planning Briefs Committee on Westminster council, Alastair Moss, said: "At present no planning applications have been submitted to us by the Mayor's office regarding the fourth plinth."

Although the Queen has been painted by a variety of artists, from Lucian Freud and 28-year-old Christian Furr to more traditional painters, there are no statues of her in the capital. The only statue in the UK is in Windsor Great Park and was produced by the Crown Estate to mark the Golden Jubilee.

There are, however, several statues of the Queen on horseback abroad. In Ottawa, the capital of Canada, the Queen is depicted on her horse, Centenial, the former Royal Canadian Mounted Police horse officially presented to her in 1977. The monument, unveiled as part of Canada's 125th anniversary celebrations, was created over two years by the artist Jack Harman and a staff of 10 others.

In Saskatchewan, the sculptor Susan Velder portrayed her riding her favourite horse, Burmese, with a bronze statue in honour of the 50th anniversary of her reign. The statue, which features a black Canadian mare ridden by the Queen on 18 consecutive birthday parades, was unveiled by the monarch in 2005.

As well as its centrality, another reason why the Trafalgar Square plinth is seen as the best spot for a statue honouring the Queen is that water saturation levels in London mean there are few locations that are suitable for prestigious new statues.

One senior Whitehall source suggested that, with the plan now in the public domain, the authorities should erect the fourth plinth statue now, while the Queen is still alive. "She certainly deserves it," the source said.