The cost of renovating the British parliament's clock tower, which houses Big Ben, has more than doubled to £61 million (69 million euros, $82 million), authorities said Friday.

Works on the Elizabeth Tower, one of the world's most famous landmarks, are "more complex" than previously thought, the House of Commons and House of Lords commissions announced.

The Big Ben bell, whose famous bongs have rung out across London since 1859, fell silent in August as scaffolding began going up the side of the 96-metre tower.

When it reaches the top, work will begin on the clock, the roof, the interiors and the brick and stonework.

The entire Houses of Parliament -- a UNESCO World Heritage site -- needs renovating, but the tower requires urgent attention.

In a statement, the commissions expressed their "disappointment" that the costs had spiralled from the £29 million estimated only last year.

Works will start with the renovation of the Ayrton Light on top of the tower, which shines to show that parliament is sitting, and the refurbishment of the cast iron roofing.

The clock will be dismantled piece by piece for cleaning and repairing, although one of the four faces will always be visible, driven by a modern electric motor while works are carried out on the original mechanism.

The commissions announced that construction firm Sir Robert McAlpine Ltd, which already had the contract to put up the scaffolding, would carry out the works.

The increased costs do not bode well for renovations of the wider estate, which has leaky roofs, is riddled with asbestos -- and mice -- and whose antiquated wiring and heating systems represent a major fire risk.

The cost of renovating the building has been put at between £3.9 billion -- if lawmakers were prepared to move out during the works -- and £6 billion, according to a report published last year.afp