New and improved rail links to Heathrow are needed to cope with a surge in passengers caused by the airport's planned expansion and possible third runway, the London Assembly has been told.

While the Elizabeth line has helped widen the airport's "catchment" of passengers from across London, access to Heathrow remains hampered by a lack of rail connections to the south and west, according to senior officials at the airport and Transport for London (TfL).

The Assembly's transport committee heard at a meeting last week that proposals for a western rail link into Berkshire, and a southern rail link into Surrey, have both been developed in years gone by.

"Those [existing] rail connections into central London are great, but the lack of them in any other direction is definitely a problem," said Christina Calderato, TfL's director of transport strategy and policy.

She added: "Any [airport] expansion of that nature… means we would definitely need to be looking at significant investment, particularly in rail."

A higher frequency of Elizabeth line services, as well as a full signalling and fleet upgrade for the Piccadilly line and capacity improvements at Holborn station – a popular interchange for passengers travelling in from Heathrow – would all be necessary "to give you that extra capacity that you'd need to cope with that level of passenger increase", she said.

Marcus Jones, Network Rail's western route director, told the committee: "The business case is positive for the [proposed] western rail link, so it is a good option for us in the future."

Anthony Smith, chair of the Heathrow Area Transport Forum – a partnership of organisations working to boost accessibility to the airport – pointed out that Terminal 5 was built to accommodate new rail links.

"There's a quite extraordinary sort of hole in the wall at Terminal 5, in the underground [part of the building], where you go through this door and there's this huge, cavernous box ready to go, and provision was made for the future of extending those services," he said.

In terms of providing a rail link to the south, he said there were two options, one of which would involve extending the Elizabeth line down to Staines, "which would be an absolute game-changer". The other possibility would see the South Western Railway linked up with Terminal 5, providing "a huge range of travel options, which go way beyond the airport".

Mr Smith added: "We are waiting for the funding, we're waiting for what might happen with the future expansion of the airport or not, but I think the current rate of passenger growth argues very strongly for getting thinking about these schemes going now, because even if we all rushed out today and started digging, it would be about seven years before anything actually ran."

Sophie Chapman, the airport's surface access director, said ministers "have made it clear that there is no Government money for either of those schemes".

She added: "We support both the schemes. We need to do our work on expanding Heathrow [first], to understand what we need to achieve in terms of mode share [of passengers travelling to the airport]."

In a separate meeting last week, TfL commissioner Andy Lord said that regardless of whether Heathrow gets a third runway, the airport's public transport links will need to be improved to accommodate capacity upgrades at Terminals 2 and 5.

Heathrow believes that this project could boost the airport's overall passenger numbers to roughly 100m – a rise of 16m on last year's record-high figure of 83.9m. A third runway could then boost potential capacity further, to 140m.

"Flying the passengers in, and flying them out, is the easy bit," said Mr Lord. "Getting them to and from the airport is the really difficult bit – and there will need to be a significant upgrade of surface access to Heathrow to enable those additional passengers to be delivered."

Heathrow also faced questions from the Assembly over its reduced contribution to the cost of building the Elizabeth line, and what this could mean for future railway funding.

The airport was originally expected by the Department for Transport to chip in £230m for the line, but this was reduced to only £70m after the Civil Aviation Authority (CAA) concluded in 2014 that with Heathrow already thought to be operating at or near to capacity, the line would bring no "net benefit" to the airport.

The Elizabeth line's overall cost meanwhile increased from an estimated £14.8bn in 2010 to a final bill of £18.8bn when it opened in 2022.

Liberal Democrat member Hina Bokhari asked whether Heathrow still believes that the CAA's conclusion about the line bringing no "net benefit" to the airport was correct.

"I'm not sure it's appropriate for me to comment on the CAA's calculation," Ms Chapman said. She confirmed however that the Elizabeth line had helped to increase the overall "mode share" of passengers coming to the airport by rail or Tube, by about two or three percentage points.

Ms Chapman added that Heathrow would give the committee a written answer as to "whether there's been an increase overall in passengers, as a result of the Elizabeth line," though she acknowledged that the line had increased the airport's overall "catchment" area.

On Heathrow's contributions to public transport more generally, she pointed out that the airport spent £9m in 2024 subsidising bus and coach services – and expects to spend similar sums in future years.

The committee's Labour chair, Elly Baker, said that while this spending was welcome, it pales in comparison to the investment which was being discussed for the Elizabeth line.

"When we're looking at hundreds of millions to improve the Elizabeth line – that may or may not have delivered improvements to your business, but that you didn't have to pay – I think that's really important considering the context we're in, in terms of improvements that may be required in the future," Ms Baker said.

"We'll come back to you on that," Ms Chapman replied.