The six-bedrooom apartment at One Hyde Park, Knightsbridge, stretches across two floors and boasts bullet-proof windows, a panic room and views across the Serpentine.

The new owners – who have already exchanged contracts – will also have access to 24-hour room service from the neighbouring Mandarin Oriental hotel, and protection from SAS-trained security guards.

The £140 million price tag makes the penthouse the most expensive property in Britain, passing the £115 million paid for a flat at the exclusive 8 St James’s Square development in Westminster in 2008.

While the identity of the buyers remains a closely-guarded secret, estate agents specialising in high-end London properties have reported a recent wave of inquiries from the Middle East and Nigeria, on the back of rising oil prices.

One Hyde Park, which boasts 86 properties starting at around £20 million, is the brainchild of luxury property developers Christian and Nick Candy, whose bid to redevelop the Chelsea Barracks collapsed after the Prince of Wales raised architectural objections with their Qatari partners.

With the final touches to the four hexagonal tower blocks that comprise One Hyde Park due to be completed by December, The Daily Telegraph understands that 65 per cent of the flats and penthouses have now been purchased.

Trevor Abrahamson of Glentree Estates, one of the subagents for One Hyde Park, said that super-rich buyers had returned to the London market in droves over the past six months, sending “trophy” home prices to new highs.

Meanwhile, the overall housing market has begun to fall again after months of recovery, according to figures released yesterday by the Royal Institute of Chartered Sureveyors.

Mr Abrahamson said: “London is the capital of capitals. Even international businessmen with no interests in London want to have a home here.”