Loseth told the company’s board that he does not want to negotiate an extension of his fixed-term contract, which is due to expire in March 2015.
In a statement, the company said it has already started looking for his successor.
Loseth (pictured) – who has been in the job since 2010 – said his decision to step down was “for personal reasons”.
“I have notified the board early in order to provide clarity and to give them sufficient time for recruitment. I shall remain fully committed to my work at Vattenfall until the end of my contract.”
Vattenfall had planned to begin negotiations over an extension to Loseth’s contract this month.
His decision comes as Vattenfall reported an operating profit of nearly SEK 21bn ($3.2bn) for the period January-September of this year.
The company – best known for its wind business but also highly active in nuclear, biomass, coal, gas and hydro – hailed the profit as “stable” and said it was achieved thanks to its cost-cutting measures and price hedging.
The figure was revealed as Vattenfall released its third quarter results, which saw net sales rise by 9.9 per cent to SEK 36,997m ($5.8m) for the quarter, while operating profit was SEK 4,818m.
Loseth said that “wholesale electricity prices continue to be weak, which is putting pressure on profitability for Europe’s power producers”.
However, he added that “thanks to cost-cutting and previously contracted price hedges, Vattenfall is able to report a stable underlying profit”.
He said Vattenfall was also “showing strong cash flow after investment”.
He added that plans to splitt Vattenfall’s operations into two regions – the Nordic countries and Continental Europe/UK – is “progressing according to plan, and the new organization will take effect on 1 January 2014”.
Loseth said the new geographic structure “will improve our opportunities to meet the varying conditions and challenges in the respective markets”.