British energy supplier SSE said today that annual earnings fell more than a third as it battled stiff competition.

The company, which owns SSE Airtricity here, also warned of an uncertain outlook due to the UK Labour party's plans to renationalise Britain's energy networks. 

A British election is not due until 2022 and opinion polls show that the main opposition party is falling far short of a majority.

But Labour laid out plans this month to offer shareholders less than current market value under a future renationalisation. 

"There is a huge degree of uncertainty, including what the final form of Labour's policy will be about the final content of any legislation; and uncertainty about how and when any legislation would be implemented," SSE said in a statement. 

The company, one of Britain's "big six" energy suppliers, said adjusted earnings for the 12 months ended March 31 fell by just over a third to 67.1 pence per share. 

Analysts on average were expecting earnings of 69.4 pence per share for the reported year, according to IBES data from Refinitiv. 

SSE said the earnings did not include its Energy Services division which is held for disposal following the collapse of a merger with peer Innogy's npower last year.

Profits at the Energy Services business were hit by a regulator-imposed price cap on the most widely used energy tariffs, SSE said.

UK energy regulator Ofgem was told by parliament last year to cap energy prices after lawmakers said customers were being overcharged for electricity and gas. 

Prime Minister Theresa May had called the tariffs a "rip-off".

SSE said that customer numbers at its Energy Services business also fell 8% to 6.25 million.

Large energy suppliers in Britain have struggled against competition from smaller, nimbler rivals often able to offer cheaper prices.