The Royal Air Force are drafting in aircraft over mounting fears in Derbyshire for a reservoir on the brink of collapsing.
A Chinook was brought in on Friday morning after hundreds of Whaley Bridge residents were evacuated due to an 'unprecedented, fast-moving, emergency situation" following the week's heavy rainfall.
Toddbrook Reservoir - which contains around 1.3 million tonnes of water - has seen "extensive" damage during the flooding and images appear to show a huge hole in the dam wall.
The helicopter was sent from RAF Odiham in Hampshire to assist at the Reservoir, a Ministry of Defence spokesman told PA.
He added: "It will drop one-ton bags of aggregate - a mixture of sand, gravel and stone - into Todd Brook.
"This is intended to stem the flow of water into the reservoir."
Videos shared by Shirebrook Fire Station showed the Chinook laden with the aggregate as it flew above the area and hovered above the the dam wall.
Derbyshire Police said late on Thursday that 400 tonnes of aggregate would be brought by the RAF as part of a multi-agency taskforce.
Speaking on Thursday evening, Deputy Chief Constable Rachel Swann, chairwoman of the Local Resilience Forum, said: "At this time the future of the dam wall remains in the balance and I would remind people of the very real danger posed to them should the wall collapse."
Firefighters deployed from across the country used at least 10 high volume pumps to reduce water to a safe level before work will begin to repair the dam wall.
Many people were told to leave their homes and directed to an evacuation point at a school in Chapel-en-le-Frith.
Police added that a timescale for people to be able to return to their homes is "currently unknown".
Richard Parry, chief executive of the Canal and River Trust, which runs the reservoir, warned it could be "at least 24 hours" until they can rule out the dam collapsing.
"We clearly don't know the nature of the failure, we've not had the opportunity to examine it, but we're operating in a very precautionary way with the other agencies," he told BBC Newsnight.
"Our first priority is to draw down the water and it's very important that we do keep everyone out of the area until that is done.
"It will be at least 24 hours, it could be longer, it really depends on how much progress we can make overnight and into tomorrow morning."
He added that the last annual inspection of the structure by a senior engineer was last November.
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.
A small number of properties in the areas of Furness Vale and New Mills, outside Whaley Bridge but inside the flood risk area, were also evacuated on Thursday evening.
A local resident told PA that another section of the spillway - designed to release water - further collapsed on Thursday evening.
Carolyn Whittle, who lives in Meadowfield, on the hillside in Whaley Bridge, said: "Another section of the concrete on the dam face has now collapsed."
The 45-year-old, who works for GM Moving, said: "I've lived in Whaley (Bridge) for the best part of 45 years, and I've never seen water flood over the dam like that, ever, nor thought that we could possibly be at risk in this way."
Rail passengers on the Liverpool Lime Street / Nottingham / Norwich line had their journeys disrupted as a result of the dam threatening to collapse.
East Midlands Trains said: "This rail closure of the Hope Valley follows a request by police in relation to the damaged dam at Toddbrook Reservoir, Whaley Bridge.
"We are working closely with other agencies to enable the railway to be re-opened as soon as it is safe to do so."
Forecaster Luke Miall said showers in the area had eased overnight, though there was a possibility of rain later in the day.
He added: "There is still a risk of showers breaking out in the afternoon, but it's a predominantly dry picture for Friday."
Sporadic rain was also likely in north west Scotland and south west England.