State regulators were told more than a decade ago that Oroville Dam in Northern California was at risk of collapsing and unleashing billions of gallons of water onto neighbouring towns.

The flood warnings were dismissed with officials arguing that the construction was stable and did not need to be reinforced with concrete – a decision which could prove catastrophic as the dam is on the brink of collapse.

Nearly 200,000 people living near Oroville – 150 miles north-east of San Francisco – were ordered to evacuate on 12 February after a massive hole was found in the dam's emergency spillway.

Residents have not been allowed to return to their homes as more rain is forecast and authorities fear that the hole in the spillway could rupture and send raging floodwaters down the Feather River, devastating thousands of houses along the way.

It has now emerged that federal regulators did not take enough measures to prevent the unfolding scenario. In 2005 environmental activists and local campaigners warned officials about the risk of flooding and asked them to reinforce the hillside with concrete and prevent erosion from allowing water to escape, according to San Jose Mercury News.

Three environmental groups – Friends of the River, Sierra Club and the South Yuba River Citizen's League – told state officials that the design of the emergency spillway "could not only cause additional damage to project lands and facilities but also cause damages and threaten lives in the protected floodplain downstream."

Ron Stork, a senior policy advocate with Friends of the River, said the groups had concluded that "the Oroville Dam complex could not be used safely or confidently to conduct flood control operations." Following the evacuation order, Stork told reporters that the campaigners had demanded state officials to build "a proper spillway."

He described the current emergency spillway as "a little bump on the top of a hill" and said that without proper concrete reinforcement the hillside was susceptible to erosion damage.

"The warning that was given should have been taken with the utmost seriousness," said Bob Wright, an attorney at campaign group Friends of the River.

California Governor Jerry Brown wrote a letter to President Trump on 13 February, asking him to "issue an emergency declaration for direct federal assistance for the counties of Butte, Sutter and Yuba, as a result of the potential failure of the Lake Oroville Dam emergency spillway."