The firm constructing the HS2 high-speed rail line has confirmed that it is investigating the emergence of a sinkhole close to a Buckinghamshire river, which has been identified as “quite sizeable” in correspondence seen by the Guardian. The sinkhole is close to Shardeloes Lake near Amersham, and an email sent to local residents by an official at HS2 Ltd’s contractor Align on Saturday evening stated: “It’s quite sizeable – around six meters in diameter and five meters deep. These are approximate measurements.” The sinkhole is in Bazzards Field, south-west of the lake. It is near a public right of way and has been fenced off. The email adds: “The landowners have been informed and livestock moved to a safe place.” HS2 Ltd sources said the area is above a section of completed tunnelling, so tunnelling operations are continuing. They said the Environment Agency had been notified and that HS2 was engaging with the landowner. The sinkhole is the latest problem to hit HS2 construction work. There were three separate incidents related to “bubbling pools” between February and April and other problems in Ruislip. The Birmingham to Crewe section of the track has been delayed and costs of the project are spiralling. Paul Jennings of the River Chess Association, which monitors the work of HS2 and campaigns for clean water, said he had contacted the Environment Agency after the sinkhole appeared and urged it to halt any further work until it had been investigated. He said he had also contacted Thames Water and asked them to pause supplying water to HS2 in the area until an investigation has been carried out. “We have been predicting that something like this would happen for the last 10 years,” Jennings said. “We have been giving a consistent message to HS2: ‘Don’t tunnel through chalk, it’s unstable.’” After images of the sinkhole emerged on social media anti-HS2 campaigners have been messaging HS2 on Twitter saying “Time to stop digging.” An HS2 spokesperson said: “We are aware of a small area of ground movement within a field above the Chiltern tunnels. Investigations are ongoing, but this is likely to be linked to pre-existing ground conditions above the tunnels. The site has been sealed off and there is no risk to the public.”