A pollution investigation into a sinkhole caused by the boring of a tunnel for the new HS2 railway line has been completed.

A large sinkhole appeared in Bazzards Field, south-west of Shardeloes Lake near Amersham in May.

The Environment Agency launched a probe into possible pollution caused by the 6m (20ft) wide, 5m (16ft) deep crater, which has now been filled with chalk.

The agency concluded no pollution was found near the sinkhole.

The Local Democracy Reporting Service (LDRS) has asked the Environment Agency for a copy of its report.

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An EA spokesman said: “Following an investigation of a sinkhole on private land near Amersham in Buckinghamshire, we found that there was no pollution to the local environment or permits breached.

“Our specialist officers work closely with HS2 Ltd and contractors to ensure all relevant legislation is followed.

“We also continue to talk to local people, the water industry and local authorities to ensure the environment is protected throughout the HS2 project.”

The sinkhole appeared above the 10-mile (16km) Chiltern tunnel – the longest on the HS2 line – which is more than 75 per cent complete.

The hole, which was near a public right of way, was filled in with 165 cubic metres of chalk by HS2 contractors in September.

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It took around three months to get permission from the landowner, and the first stage of the work was completed after two and a half weeks.

A HS2 spokesperson told the LDRS: “The topsoil was removed first and stockpiled. The hole was then filled in stages, with each layer compacted as they went along. Topsoil was then added.

“The second stage of the work will be done in Spring 2024 – this pause is to allow time for any further settlement over the winter.”

Paul Jennings of the River Chess Association, which campaigns for clean water, got in touch with the EA after the sinkhole emerged in May.

He said: “We have been giving a consistent message to HS2: ‘Don’t tunnel through chalk, it’s unstable’.”

However, HS2 has tried to dispel environmental concerns over the sinkhole.

A spokesperson said: “Our regular water monitoring has not shown up any impact on water quality. Important to note that the hole itself, was actually above the water table.”

Chesham and Amersham Councillor Gareth Williams, who is the local Tory candidate for MP, claimed Buckinghamshire Council had pressured HS2 to fill in the sinkhole.

In a recent statement, he said: “Long after the TV cameras have gone, we at Buckinghamshire Council have had regular meetings with HS2 to ensure progress happens in repairing the large sinkhole that emerged and that the local farmers are properly informed and involved.”

He added: “We remain opposed to the whole project due to its impact on our environment, roads and ever-expanding cost and will keep fighting for the interests of our residents.”