As Europe prepares for a cold winter due to an electricity crunch, one of Sweden's six nuclear reactors will go back online after repairs on January 31, two months later than scheduled, power company Vattenfall said on Tuesday.

Grid operator Svenska Kraftnat said the delay at Ringhals 4 in southwestern Sweden increases the "real risk" of a power shortage and "we may have to disconnect electricity consumers this winter".

Sweden will have to import more electricity to ensure a balance between production and consumption, but the country may not be able to rely on its neighbours.

Ringhals 4 was stopped for annual routine maintenance in August, but was not able to restart after the pressure vessel surrounding the reactor core was damaged during tests.

Vattenfall had initially expected the repairs to be completed by November.

Sweden is a net exporter of electricity, but supply issues have increased with the closure of several nuclear reactors in the past decade and less reliable production from renewable energies, meaning it must import power at times.

Europe's energy crisis, sparked by the drop in Russian gas deliveries following the war in Ukraine, has aggravated the situation.

Sweden, which has already resorted to firing up reserve power plants that run on oil, derives most of its electricity production from hydro power (45 percent), nuclear (30 percent) and wind power (17 percent).