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One of Japan's major energy companies is making a significant investment in at least 20 UK offshore oil and gas fields.

JX Nippon Oil and Gas Exploration is buying assets from Italian firm Eni.

The move is intended to push up its UK production from 9,000 barrels per day to 50,000 barrels by 2019.

These are not assets which Eni has been operating, but in which it has a stake. Two of the five packages of offshore assets have been completed.

Until the other parts of the deal are completed in the next few months, the companies are not putting a value on it.

At present, only some of the assets are producing oil and gas. However, when the Mariner and Culzean fields start pumping, that should sharply increase the Tokyo-based refinery and retailing firm's presence in British waters.

The company will have a 29% stake in Mariner, south-east of Shetland, which is one of the UK's largest undeveloped oil fields, reckoned to hold up to 500m barrels of recoverable oil.

Seventeen producing fields included in the Eni deal include the Andrew, Magnus, Kinnoull, ETAP, Merganser and the Flotta Catchment Area, including the Claymore field.

JX Nippon was also successful in the most recent round of drilling licences allocated by the UK government for west of Shetland.

Shigeo Hirai, president of JX Nippon Oil and Gas Exploration, said: "This is an important contribution to JX's upstream goal of global production of 200,000 barrels of oil equivalent per day by 2020.

"The acquisition of theses assets gives JX substantial long-term oil and gas production in the UK, where JX has built up considerable expertise over the past 20 years."

Malcolm Webb of Oil and Gas UK, the industry body, said the JX Nippon decision demonstrates that tax incentives negotiated with the Treasury are making UK waters a more attractive region for investment.

In recent weeks, other major asset sales by BP have seen investment by Taqa, owned in Abu Dhabi, and by Perth-based SSE, which bought a stake in the Sean gas field.