The Guardian

A thick crust of bird droppings is piled on the gilded balustrade of one of Britain's most expensive properties. Pigeon skeletons lie among shattered mirrors and water streams through broken cornicing. This is The Tower, a £30m palace in "Billionaires Row" in north London whose spectacular ruin has been kept secret until now.

It is one of 10 mansions in the middle of The Bishops Avenue - the heart of London's spiralling property market - that have stood almost entirely vacant since they were bought a quarter of a century ago, it is believed on behalf of members of the Saudi Arabian royal family. Their Grecian columns are cracking into pieces and mosaic-tiled swimming pools are filled with rubble. Nature has taken over to the extent that owls have moved in.

It is a desolate scene repeated up and down the supposedly prestigious avenue that Lloyds Bank has calculated is the second most expensive street in Britain. While more and more people struggle to get on to London's property ladder as house prices rise at 11.2% a year, the Guardian has established that 16 mansions on the most expensive stretch of The Bishops Avenue are sitting empty, many behind padlocked gates, with their windows shuttered with steel grilles and overgrown grounds patrolled by guard dogs.

Across the street from the former Saudi properties stands another derelict mansion worth £18m with smashed windows and walls coated in anti-climb paint. Metal grilles block the windows of another believed to belong to a Middle Eastern owner through a Channel Islands company which has been "sold on behalf of receivers" for £20m.

But that doesn't stop the prices going up. Dryades, a mansion until recently owned by a Pakistani politician, sold for £12m in 2007 and is believed to be worth about £30m today. Heath Lodge, the scene of the 1984 murder by silver bullet of fashion tycoon Aristos Constantinou, is worth £13m today after having been sold in the late 1970s for £400,000.

The dereliction can be agonising for people struggling to keep a roof above their heads in one of the world's most expensive cities. One security guard working on the avenue said he had only recently been homeless and it was exasperating to see so many tens of thousands of square feet of property - enough to house dozens of people - falling apart. "How do you let something like this go to waste?" he said. "You wouldn't, would you?"

Over the last half a century this curving road near Hampstead Heath has become a haven for the winners and losers of numerous geopolitical shocks. Royals flushed with oil wealth from Nigeria and Saudi Arabia were among the first to come. Iranians fled here after the fall of the shah. Now Chinese househunters are following Russians and Kazakhs who have spent millions securing an address estate agents tell them is as world famous as the Champs Elysées and Rodeo Drive. This week two mansions were on sale for £65m and £38m, promising endless Italian marble, leather-padded lifts and luxury panic rooms. Roaring Lamborghini sports cars and Bentley limousines negotiate the constant traffic of construction vehicles refurbishing mansions.

The avenue is home to Princess Al Jawhara bint Ibrahim Al Ibrahim, widow of Saudi Arabia's late King Fahd, and Richard Desmond, owner of the Express newspapers. But its hidden reality is exposed most clearly in the wrecks of the 10 mansions sold last October for a combined £73m.

Land Registry records show the portfolio was assembled between 1989 and 1993, around the time Saddam Hussein's sabre rattling in the Gulf threatened the House of Saud. The deal on the boltholes was brokered by Rafic Hariri, the future prime minister of Lebanon, who at that time was working as a Saudi envoy, sources familiar with the transaction say. Five identical mansions were snapped up for £1.5m each along with the palatial Tower - built on the site of the former home of the 1940s Hollywood star Gracie Fields - and three neighbouring mansions. But after Saddam was beaten back, the Saudis let dilapidation take hold.

The waste is spectacular. In the grounds, stone fountains crumble and lawns have become bogs. Inside the Georgian mansion water drips through a huge crystal chandelier on to a thick carpet rotting under sections of collapsed ceiling. Moss grows through shattered bricks and mirrored tiles are scattered across a bathroom. An odour of fermentation pervades neighbouring Redcroft and the only signs of life are an old and jammed Arabic Olympia typewriter and a hotel-style sign warning: "Visitors may be asked to submit to a search of person or baggage by security staff." The swimming pool is filled with a foot of brackish water and has flowers growing through its tiles. Wooden slats bulge away from the sauna. Behind