The Indian-owned company will upgrade its gasoline producing unit. At the same time, it will undertake routine maintenance of other parts of the plant, meaning that production will be largely shut down, Jon Mason said. The wider turn-around was a regular four-yearly event.

He declined to give further details on the timing of the shutdown at the refinery in north west England.

Essar UK chief executive Volker Shultz said increasing the flexibility of the crudes the plant can process would substantially contribute to improving the overall refining margin.

He said that the refinery would be able to take a wider range of crudes, because it had ceased production of lubricants.

"Lubricants, which were only 1-2 percent of production, controlled 25 percent of the crude slate," he said.

Up until February, North Sea crudes constituted three quarters of intake, but last month made up less than half, with West African, North African and Canadian crudes making up the balance.

Volker said that the aim of the upgrade was to raise overall margins by $3 per barrel over three years by using natural gas instead of oil in the boilers, as well as diversification, and that this should be achieved within around a year from now.

(Reporting by Simon Falush and Claire Milhench; editing by Keiron Henderson)