Companies worth around £100 billion have left the London Stock Exchange so far this year, either by being bought out or by moving their shares’ headquarters overseas, and experts fear many more will follow.

Research by the Evening Standard and investment bank Peel Hunt shows that companies worth over £26 billion have already agreed to be sold to other listed companies or private equity in 2024.

Add to that a total of £38 billion worth of value for companies moving their main listings overseas. And Anglo American today rejected the biggest takeover bid yet, saying BHP’s £31 billion offer “undervalued” the company and was “opportunistic”.

Including the current headline sum from this deal, the total market capitalization associated with the exodus is £95 billion.

City experts expect a higher bid from BHP. Another multi-billion dollar deal was closed today for cybersecurity company Darktrace.

Charles Hall, head of research at Peel Hunt, said: “Twenty-one companies are in the bidding process, 12 of which are listed on the FTSE 350.”

He warned: “We are exporting some of our fastest growing companies at a rapid pace and Darktrace is going to US private equity. This clearly shows how dire the state of the London market is and how urgent measures are needed to restore competitiveness and capital flow, including the abolition of stamp duty, pension reform and the introduction of the UK ISA.”

Any deal achieved for Anglo would bring the trend into the upper quarter of the FTSE 100.

Company bosses are already expressing frustration at the relatively low valuations of large companies in London, especially compared to those offered by arch-rivals New York.

The flight from the City has already reached companies in the top price bracket. Packaging manufacturer DS Smith is being offered a takeover bid of 5.7 billion pounds by International Paper.

And there could be even bigger earthquakes ahead. Wael Sawan, CEO of Britain’s largest company Shell, is said to have given London a two-year deadline to raise its valuations before the company is potentially relocated to the US.

The speculation about a takeover bid has been dubbed “the great fire sale of London” by City commentators and has helped push the FTSE 100’s valuation to record levels, although it is still below the historic highs of arch-rival New York. City experts were prepared for further action.

Tony Cross of investment database Investegate said: “At what point did Anglo’s management reach an impasse and have to either accept a hostile takeover bid or find an IPO with a more accurate valuation?”

He pointed to a “bigger question” and asked: “Why does the London market seem unable to accurately value even the largest companies?”

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Concerns that investors are buying fewer UK assets have prompted the government to pump more cash into the market to support demand for shares and lift valuations. Chancellor of the Exchequer Jeremy Hunt’s so-called Mansion House reforms, unveiled last July, and his plans for tax relief on UK ISA deposits are designed to pump more liquidity into the market.

Peel Hunt’s Hall added: “It is vital that we maintain the momentum of the Mansion House reforms and initiatives such as the British ISA to help the London capital market fulfil its critical role in serving the wider economy.”

He also noted that there was some hope for a revival of initial public offerings, in which companies raise capital for the first time through share sales.

“Our latest IPO speedometer suggests a recovery as more new companies enter the market over the next 12 months. However, more needs to be done to consolidate what could be a fragile recovery.”

Meanwhile, the mood in the City remained gloomy as another day brought another multi-billion pound takeover deal.

Dan Coatsworth of broker AJ Bell spoke of a “time of crisis” as London “fights to preserve the integrity of the British market”.