Ministers have told High Speed Two to consider running its trains at lower speeds, in an attempt to rein in the spiralling budget and begin operations as soon as possible.

HS2 Ltd will assess whether limiting the speed to 186mph (300km/h) instead of 224mph could save money – potentially billions of pounds – and bring the railway into being earlier in the 2030s.

Most fast trains in the UK run at a maximum of 125mph, while high-speed trains to Kent and the Channel tunnel using the HS1 line run at up to 186mph, the typical European maximum.

In an update to parliament, the transport secretary, Heidi Alexander, said that since the HS2 chief executive, Mark Wild, gave his initial findings for a planned “reset” of the high-speed rail scheme’s timeline and budget, “the scale of the challenge has become even clearer”.

She said Wild’s work so far showed that HS2 Ltd “did not have an accurate assessment of how much work had been delivered, or of how much was left to do. It is now clear that previous plans significantly underestimated the work required.”

Wild, who took over as the chief executive of HS2 Ltd in late 2024, is understood to have delivered a first review of costs and a new proposed work schedule to the Department for Transport.

However, Alexander said she had now commissioned Wild to report back before summer recess on the possible savings from slower trains.

She said that no railway in the UK was currently engineered for 360 km/h, adding: “This means that the project would have to wait for HS2 tracks to be built before testing any trains – an approach which could increase costs and delay the completion of the project. The alternative would have been to send trains abroad to test.”

Alexander praised Wild’s leadership and said HS2 was now “working”, meeting construction milestones including completing excavation of all 23 miles of deep tunnels needed for the opening stage of the railway.

The government hopes to hammer down the price before it publishes the full reset plan, including an overall budget restated in 2026 prices. After several years of soaring inflation during Covid, with labour and steel costs rising sharply, the figure is widely expected to surpass £100bn.

Monday’s six-monthly update put the total expenditure to date at £46.2bn, at current prices – including £2.6bn spent on the northern leg of HS2 from Birmingham to Manchester, which was axed by Rishi Sunak in 2024.

Government sources said that the speed of the trains and associated costs showed the “gold-plating” and “needlessly overspecced” design for what would be the fastest railway in the world, drawn up by the previous Conservative government.

Wild said: “I made a commitment to the transport secretary that I would regain control of HS2 and bring an end to the project’s cost increases and delays.

“With performance moving in the right direction, driven by the hard work of 30,000 people on the ground, we are rightly exploring options to create further efficiencies.

“Speed has never been the primary objective. This railway will deliver better journeys, more capacity on the network, and economic growth – all of which are vital to the country’s future prosperity.”