HS2 Chiltern Tunnel sinkholes appear in Buckinghamshire

The holes were discovered on land on the outskirts of South Heath, near Great Missenden, Buckinghamshire

Sinkholes thought to be caused by the boring of tunnels for the HS2 rail project have opened up in a field.

Two new holes were discovered on land on the outskirts of South Heath, near Great Missenden, Bucks, where the Chiltern Tunnel is being constructed.

HS2 Ltd said "safety was its top priority" and both sinkholes had been fenced off.

The Environment Agency said it was investigating whether any pollution had been caused.

It means four craters have been caused by HS2 in the county in nine months.

The smaller of the two holes was found next to Frith Hill during routine monitoring on 7 February, with a larger feature found in the same field five days later, according to the Local Democracy Reporting Service.

There was no public right of way across the affected land, HS2 Ltd said.

HS2 Ltd said the sinkholes had been sealed off

A sinkhole that measured about 6m (20ft) in diameter and 5m (16ft) deep opened up by Shardeloes Lake, near Little Missenden, in May, followed by another in Hyde Heath in November.

Both have been filled in and were found to have no significant impact on water quality.

HS2 Ltd said the latest sinkholes may have been caused by disturbance from tunnelling and recent rainfall that exposed existing weak spots.

"We have always recognised that the geology of this part of the route is particularly challenging - with poorer quality, less stable chalk susceptible to ground movement," a spokesman said.

"Temporary fencing has been installed and extra monitoring put in place."

The Chiltern Tunnel is the longest on the HS2 project

The 10-mile (16km) Chiltern Tunnel is the longest on the HS2 project, which aims to improve links between London and Birmingham.

An Environment Agency spokesman said: "We are investigating sinkholes on private land at Frith Hill in Buckinghamshire to understand whether they have resulted in any pollution to the local environment.

"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."

Buckinghamshire Council has been approached for comment.

Previously, a sinkhole opened up by Shardeloes Lake, near Little Missenden