As Russian gas cut-offs upend European energy security, the continent is struggling to cope with what experts say is one of its worst-ever energy crises -- and it could still get much worse. 

For months European leaders have been haunted by the prospect of losing Russia’s natural gas supply, which accounts for about 40% of European imports and has been a crucial energy lifeline for the continent. That nightmare is now becoming a painful reality as Moscow slashes its flows in retaliation for Europe’s support for Ukraine, dramatically increasing energy prices and forcing many countries to resort to emergency plans, and as backup energy suppliers such as Norway and North Africa are failing to step up.

“This is the most extreme energy crisis that has ever occurred in Europe,” said Alex Munton, an expert on global gas markets at Rapidan Energy Group, a consultancy. “Europe [is] looking at the very real prospect of not having sufficient gas when it’s most needed, which is during the coldest part of the year.”