This article is more than 1 year old

This article is more than 1 year old

The prime minister has visited the Derbyshire town of Whaley Bridge, where at least a thousand residents were forced to flee their homes amid fears a dam could collapse, engulfing the community of 6,500 people.

Boris Johnson met a number of families affected by the evacuation. He told them he had flown over the dam twice and it was “dodgy but stable”, but police warned there was still “a substantial threat to life” if the dam wall fails.

Water levels at the Toddbrook reservoir have been reduced by half a metre following extensive pumping but engineers remain “very concerned” about the integrity of the damaged 180-year-old structure, which contains around 1.3m tonnes of water.

Throughout Friday, an RAF Chinook dropped 200 tonnes of aggregate on to the damaged section of the dam, which experts still fear could collapse.

Derbyshire police made the “difficult” decision to allow some residents home for short, chaperoned visits on Friday afternoon to pick up essential items and pets, said assistant chief constable, Kem Mehmet. “This is very controlled, I must stress that, because this is still life at risk,” he told reporters.

The number of people returning will be restricted to one person per household, he said, and added that it was “difficult” to say when people would be allowed to return permanently.

Julie Sharman, chief operating officer for the Canal and River Trust, told a press conference the water level needs to be reduced by several more metres to allow engineers to assess the damaged section.

“This is still a very critical situation,” she said. “Until we are confident we can control that risk, then our position has to be to protect the public safety and limit access because we don’t want to put people at risk.”

Improving weather and work on the inflows means that the amount of water entering the reservoir has reduced.

Police have closed railway lines in the Whaley Bridge area due to the risk of flooding.

Play Video 1:31 RAF helicopter joins effort to stop Derbyshire dam collapse – video

On Friday evening the prime minister visited Chapel-en-le-Frith high school, which has become a muster point for stranded residents and the emergency services. Talking to residents, he said: “I flew over the dam and it looks pretty scary. I can see the problem.” He assured them “you will all be properly housed” if the dam burst, and told police officers: “Let’s hope it doesn’t happen.”

The Environment Agency issued a “danger to life” warning covering the River Goyt on Thursday, as the river could “rise rapidly” due to water rushing in from the reservoir.

Meanwhile, clean-up operations are under way across parts of the north-west hit by heavy rain, including Poynton in Cheshire, where residents were evacuated on Wednesday night.

Lyme Park, a National Trust estate near Whaley Bridge where the BBC’s 1995 adaptation of Pride and Prejudice was filmed, was closed on Friday after flooding caused extensive damage. Aerial photos showed parts of the estate under significant amounts of moving water, while paths, walls and fences were “washed away” as drainage pipes were exposed.

The Environment Agency has 10 flood alerts, six flood warnings and one severe flood warning in place across England.

Richard Parry, chief executive of the Canal and River Trust, said an annual inspection of the Toddbrook reservoir by a senior engineer took place last November. The reservoir is on the north-west edge of the Peak District National Park and was built in 1831, according to experts, although the Environment Agency records it as being built in 1840-41. According to a 2011 Environment Agency report on national dam incidents, Toddbrook “has a history of leakage”.