The UK’s largest airport London Heathrow catered for six million passengers in August as its recovery was held back by its self-imposed cap on passenger numbers.

Heathrow faces further turmoil after investors accounting for 35 per cent of the airport said they are selling up.

Their decision to quit follows the sale of its 25 per cent stake in the airport by the largest shareholder, Ferrovial.

At the end of last year Ferrovial, the Spanish construction and infrastructure giant that has held the largest stake in Heathrow since the 2006 acquisition of the old privatised BAA group, said it would be selling its holding for £2.37 billion. It had previously written down the value of its holding in Heathrow to zero.

Heathrow is Britain and Europe’s busiest airport. Last year it handled 79 million passengers, just shy of its pre-pandemic level of 81 million. But it has not paid a dividend in four years and is creaking under £16 billion of debt. Plans for major expansion through a third runway were quashed by politicians.

Yet Ferrovial’s stake was picked up by Ardian Capita, the French firm formerly known as Axa Private Equity, which took 15 per cent with the other 10 per cent going to Public Investment Fund (PIF), the sovereign wealth fund of Saudi Arabia prominent in the UK as the owner of Newcastle United FC; it is also as a major shareholder in Aston Martin, the carmaker.

Ferrovial’s decision to sell has triggered a so-called “tag-along” clause which allows other Heathrow shareholders to sell up too. Crucially, such tag-along rights mean minority shareholders have the right to the same price as that achieved in the initial transaction.

It has now been confirmed that three other shareholders speaking for 35 per cent of Heathrow have exercised their tag-along rights.

It is believed those shareholders are the Canadian investor Caisse de dépôt et placement du Québec, the Singaporean sovereign wealth fund GIC, and the Brisbane-based Australian Retirement Trust.

Under the agreement, Ardian and PIF have first rights to acquire all or part of the 35 per cent of shares now available at a price of £3.3 billion.

It is understood PIF does not want to increase its stake beyond 10 per cent. It is further understood that Ardian does not want to increase its stake above 25 per cent, as to do so would probably attract unwanted political and media interest.

Under the terms of the tag-along arrangement, a failure to find a buyer for the new 35 per cent stake that has come on to the market could halt or unwind the Ferrovial deal with Ardian and PIF.

The situation effectively means that for Ardian to cement its new stake in Heathrow it now has to either acquire the 35 per cent stake — which it doesn’t want to do — or find buyers prepared to part with an aggregate £3.3 billion for the shares.

It seems unlikely that Heathrow’s other remaining shareholders would want to buy more of the airport at a premium price. The remaining 40 per cent of Heathrow is owned by the sovereign wealth funds of Qatar and China and the UK academics pension fund Universities Superannuation Scheme.

The change in ownership would represent one of the biggest shake-ups in Heathrow’s boardroom since it was privatised under the Thatcher government in the Eighties.

One City source said: “Heathrow is on a strong trajectory to recovery and compared to how it has been run, under new management there is plenty of runway to make the business more efficient.”

John Holland-Kaye, Heathrow’s longstanding chief executive, stood down in the autumn and was replaced by Thomas Woldbye, who previously ran Copenhagen airport.

The world’s two largest infrastructure investors, BlackRock and Macquarie, would probably face competition issues if they were to get involved.