The Grand Staircase in No 10 is being propped up to stop movement, the Standard can reveal.
The carpeted stairs are an iconic part of Downing Street.
Presidents, prime ministers, royals and other visitors to Downing Street have been photographed walking up them.
The staircase includes black and white portraits of Britain’s former Prime Ministers, starting with Sir Robert Walpole at the bottom and with the inhabitants of No 10 going up in chronological order.
Westminster City Council recently granted listed building consent for a metal angle to be installed to the internal stone staircase “to prevent further movement”.
The local authority stressed that to “protect the special architectural or historic interest of this listed building” all new work and improvements “must match existing original adjacent work in terms of the choice of materials, method of construction and finished appearance”.
World leaders who have been photographed on the staircase include George W Bush, who was pictured in June 2008 with then Prime Minister Gordon Brown.
Tony Blair, who had stood down after facing growing pressure from Brownite MPs, can be seen in the background of the photo of Mr Bush and Mr Brown.
The application for listed building permission for the work submitted in August was treated as “confidential” by Westminster planners.
Historic England and the Westminster Society were consulted on it.
The support work is understood to be for just one step on the staircase, with its cantilevered design, and that it is precautionary rather than an emergency intervention.
Sir Keir Starmer is yet to have his portrait added at the top of the staircase, as happens to premiers after they have stepped down or been ousted.
There has been a flurry of Tory Prime Ministers in recent years including Theresa May, Boris Johnson, Liz Truss, who resigned just 45 days after taking office, and Rishi Sunak after David Cameron quit over the Brexit debacle for his party.
The staircase is one of the most recognisable areas of No 10 which has been home to British Prime Ministers since 1735.
It was designed by architect William Kent in the 1730s when he was tasked with joining three houses to form No 10 Downing Street.
Sir Winston Churchill is the only former PM to have two portraits on the stairway.
It was renovated in the 1960s, with new lamps added, to reflect the original Kent design.
At its lower end there are now photos from past Cabinets and Imperial Conferences.
