A British exit from the European Union would be severely damaging for both parties and like a divorce, the head of one of the biggest insurers and chairman of the influential Bilderberg group has warned.

Henri de Castries, chairman of Axa, said a so-called Brexit would “diminish both the status and influence of the UK” and leave the country as a “big Switzerland”.

In a wide-ranging interview, de Castries also compared the recent slide in the Chinese stock market to the Wall Street Crash of 1929 and said the private sector had a decisive role to play in tackling climate change.

De Castries is a key voice in the debate about Britain’s future in the EU given Axa is one of Europe’s biggest companies and he is chairman of the secretive Bilderberg group of political and business leaders from Europe and North America. The group also includes the British chancellor, George Osborne, the Google chairman, Eric Schmidt, and Douglas Flint, chairman of HSBC.

De Castries said the EU had provided “growth, peace, social stability” over the last 70 years but that “every rational person understands that there are flaws in the model and there are shortcomings”.

He said: “Everybody recognises the fact that there are things which need to change, but after all isn’t it the Brits who invented ‘don’t throw the baby out with the bathwater’? So don’t throw baby Europe out with the bathwater.

“There are very positive reasons to think that even if the baby is still not fully satisfactory, there are things which can be improved. It is better to face the challenges of this next century together rather than being like the former Gallic tribes trying to face the Romans one by one.”

De Castries held talks with Osborne last month when the chancellor visited France as part of his push to reform the EU. The Axa boss added: “I think this country has joined Europe for much more than pure economic reasons, not being part of it would clearly at the end of the day diminish both the status and influence of the UK.

“It would make the UK a big Switzerland. Switzerland is pretty prosperous but I would say it never had the ambition of being a model for the world.

“It’s like a divorce. Of course it would be severely damaging. But rather than looking at that, I prefer to look at the half-full bottle. What I would like to see is more people explaining pretty vocally why it is good for the UK to stay, rather than saying why it would be pretty bad to leave.”

De Castries also said that a fall in Chinese share prices was “more worrying” than the Greek debt crisis and that it could hit household incomes in China.

“The number of households who have participated in the market is not insignificant,” he said. “Nobody knows exactly, but people say 100m probably. So the number of people affected by a very sharp decline [in share prices] is not an insignificant number.

“The key question for me is to what extent is this going to affect Chinese consumer spending, because a large part of the world’s growth relies on Chinese consumer spending. Are the Chinese households who are buying German cars or American cars going to stop buying them because of lost money in the stock market? I would be surprised if it had no influence. I am sure it will. The optimistic people are saying: ‘Well, not as bad as New York in ’29.’ I hope so, we all hope so.”