The “real challenge” for ensuring HS2 trains reach London’s Euston is the development of the station, a Government adviser has warned.

Sir John Armitt made the comment a week after Chancellor Rachel Reeves announced funding to build a pair of tunnels stretching 4.5 miles between Old Oak Common – in the western suburbs of the capital – and Euston.

Speaking at the Rail Industry Association’s annual conference in central London, Sir John, who chairs the National Infrastructure Commission, said: “What was very welcome news was the announcement that they would fund the completion of the tunnels through to Euston.

“That’s the relatively straightforward bit.

“The real challenge, as everyone in this room knows of course, is Euston station, which will take longer to do than it will to drive the tunnels.

“So what’s really needed is to find a solution to dealing with Euston station.”

In October last year, then prime minister Rishi Sunak announced that extending HS2 from Old Oak Common to Euston was reliant on private investment.

This was aimed at saving £6.5 billion of taxpayers’ money.

Major HS2 construction work at a site alongside the existing Euston station has been halted since March 2023 due to funding doubts.

Sir John went on to state that the speed and route chosen for HS2 has added to its cost, as it has resulted in a large amount of tunnelling.

He said: “I would argue, personally, I think the 400 kilometres per hour (250mph) was wrong, putting it bluntly.

“Of course if we’d gone for a lower speed we’d have had different routes, and that different route might well have had less cost because it didn’t go through the Chilterns.”

He said decisions made in planning major infrastructure projects are “the most critical”.

He added: “Once you start building, there’s not a lot you can change, so you’ve got to get it right before you start building.”

The design speed for HS2 tracks is 400km/h (250mph), and the trains will have a maximum speed of 360km/h (225mph).