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Oslo: Britain and Norway have finished laying the world's longest subsea power cable, which will send wind and hydro energy between the two countries, Norwegian power grid operator Statnett announced Tuesday.

The 720-kilometre-long (447-mile-long) North Sea Link - all but four kilometres of it underwater - links Suldal in the southwest of the Scandinavian country to Blyth, near Newcastle.

The cable will deliver British wind energy to Norway, which will send hydropower to the UK in return, with testing set to start October 1.

The project is estimated to have cost between 1.5 billion and 2.0 billion euros ($1.8-$2.4 billion).

"When the wind blows in England and wind power production is high, we in Norway will be able to buy cheap electricity from the British and leave the water in our dam reservoirs," said Statnett's project manager Thor Anders Nummedal.

"When there is little wind and a greater need for electricity in England, they will in turn be able to buy hydroelectric power from us," he said in a statement.

The power capacity of the new cable is 1,400 megawatts.

The coupling of the two sections, built simultaneously from the British and Norwegian sides, took place late Monday evening.

The construction had its share of technical challenges, including the need to build a special barge to run the cable under a Norwegian lake and the drilling of a 2.3-kilometre tunnel.

"This is an important cooperation between the UK and Norway to make the most of our joint renewable energy resources," said Nigel Williams, project director at UK operator National Grid, which, like Statnett, owns 50 percent of the project.

The cable takes the crown of the longest underwater cable from Nordlink, which was inaugurated only last month and connects Norway and Germany, measuring 623 kilometres, with 516 kilometres of it underwater.

Already connected to France, Belgium, the Netherlands and Ireland, the UK is planning further direct connections with continental countries, including a 765-kilometre link with Denmark with a 621-kilometre stretch underwater.