Sir Clive Cowdery ruled on the downside of the £8.3 billion sale of insurer Resolution Life to Japanese rival.

British tycoon Sir Clive Cowdery could be in trouble after selling his insurance business to a Japanese rival.

The businessman will remain chief executive and chairman of Resolution Life after an £8.3 billion deal with Osaka-headquartered Nippon Life.

The sale could net him a fortune – possibly billions – although the exact figure remains a mystery because the size of his stake is unknown.

The deal comes amid a number of takeovers around the world including a recent £3.6 billion takeover by British insurer Aviva for smaller, struggling rival Direct Line.

Only this week saw US media firms Omnicom and Interpublic agree to a £10bn deal while Cadbury boss Mondelez rejected its bid for rival chocolate maker Hershey

Founded by Cowdery in 2008, Resolution Life has around £67bn of assets under management and over 4m insurance policies. It grew by seizing books of life insurance policies from around the world from insurers.

The businessman also founded the left-leaning think-tank Resolution Foundation, which says it is focused on improving the lives of low-to-middle income households.

The research team calculates the Living Wage, which is an hourly wage above the Government’s official minimum wage.

Cowdery was honored in 2016 for services to children and social movements.

He is also the publisher of the current affairs magazine Prospect, which is run by former managing editor Alan Rusbridger.

As director of Britain’s best bids until 2021, Cowdery helped lead a campaign to block Brexit.

Since this is a private business, the amount of his balance in the Resolution has not been disclosed. But he may be set for a big payday from a contract worth $8.3 billion.

Nippon Life, which holds a 23 per cent stake, will buy the rest of the shares for £6.4 billion.

It plans to make the Bermuda firm a wholly-owned subsidiary by the end of 2025.

Other investors include Blackstone, the US private equity firm, which will remain investment management partner after the takeover.

Cowdery said: ‘Combining the strengths of Resolution Life, with Blackstone’s investment management expertise and a well-funded parent, gives us the opportunity to accelerate growth and meet the needs of policy holders in the decades ahead.’

Nippon will also acquire the 20 per cent stake it does not own in MLC Life from National Australia Bank for £250.6million and merge it with Resolution Life Australasia to form Acenda, a life insurer.

It was the largest overseas acquisition by a Japanese insurance company.