Labour leader Ed Miliband has warned that Britain is "sleepwalking" towards a national crisis because of recent extreme weather conditions caused by climate change.

The leader of the opposition said heavy storms and floods that have devastated many parts of the UK for over a month should serve as a wake-up call on climate change and prompt a national consensus on the issue.

"We have always warned that climate change threatens national security because of the consequences for destabilisation of entire regions of the world, mass migration of millions of people and conflict over water or food supplies," he told the Observer.

"But the events of the last few weeks have shown this is a national security issue in our own country, too, with people's homes, businesses and livelihoods coming under attack from extreme weather. And we know this will happen more in the future."

Miliband said that scientific evidence of the link between climate change and extreme weather was "clear", and maintained that denial or hesitation on the issue would damage the country.

"Denial is damaging because it means you won't take the steps necessary, but dither is damaging too, because it means you are half-hearted about taking the necessary measures," he said.

Miliband added that more money will have to be spent on flood defence, and called on Conservative and Liberal Democrat factions of the coalition government to bridge their political differences and unite on the issues of climate change and extreme weather patterns.

Prime Minister David Cameron's 2010 claim of being the "greenest government ever" came under fire from his Labour counterpart, who criticised him for apparently backtracking on his commitment to the environmental cause.

"It is pretty extraordinary that it has gone from a core conviction, a part of his irreducible core, to a matter of conscience as to whether you believe it or not," Miliband said.

The Environment Agency has maintained 16 severe flood warnings – meaning a risk to life – across the UK.

Despite weather forecasts predicting less rain and lighter winds, parts of southern, south-west and central England are still at risk of flooding due to higher river levels caused by recent heavy rainfall.