Significant uncertainties remain over key aspects of the proposed expansion of Heathrow Airport, including the financing of a crucial southern rail link and the airport’s ability to attract more domestic flights, a parliamentary inquiry has found. The inquiry took place as Heathrow’s owners and proponents of an alternative third runway plan were questioned by the House of Commons Transport Select Committee on Tuesday.

The £33 billion expansion, intended to increase capacity by up to 66 million additional passengers annually beyond the current 80 million, hinges on improved transport links, including a new railway connection to the south. This link, known as the Heathrow Southern Railway, would connect the airport to the South Western Railway line into London Waterloo, but no concrete funding arrangements have been confirmed.

Helen Elsby, Heathrow’s chief development officer for the third runway project, acknowledged that the airport’s financial plans cover the construction of new airport infrastructure such as stations to accommodate the railway, but not the development of the railway line itself. The estimated cost of the new eight to ten-mile rail link is around £1.7 billion, and Elsby indicated this expense would likely fall to the government or a yet-unknown private investor. “The airport would contribute to financing the line only appropriate to the amount to the benefit of the airport,” she said.

Both Heathrow’s current expansion plan and the rival Heathrow West proposal, supported by property entrepreneur Surinder Arora and involving a shorter, less expensive third runway, stated that they had no plans to fund the southern rail connection directly.

Another issue raised by MPs concerned Heathrow’s role as a hub for domestic flights connecting UK regional airports to international long-haul services. Heathrow’s chief executive, Thomas Woldbye, conceded that the current airport does not sufficiently serve this function due to congestion and slot limitations. “It suffers from not having enough [domestic] slots. We have outsourced to Schiphol [in Amsterdam],” he said, referring to the fact that many UK domestic carriers route passengers via Amsterdam rather than Heathrow.

Woldbye expressed confidence that the expanded airport would find ways to increase domestic connectivity, stressing the shared interest of airlines and regional airports in achieving this. However, Carlton Brown, chief executive of the Heathrow West project, noted the competitive challenge posed by Schiphol’s lower fees. “Airlines from the UK regions use Schiphol because it is cheaper, so we would have to get as cheap,” he said. Arora added that while new capacity is being created, the airport cannot control airline choices.

The parliamentary session also highlighted uncertainty over regulatory financial returns, a key factor delaying Heathrow’s expansion timetable, which has been pushed back to 2039 at the earliest. Woldbye revealed that the airport’s owners—including international investors such as sovereign wealth funds from China and Gulf states—are awaiting clarity from the Civil Aviation Authority on the allowed financial returns before committing fully to the plan.