Peter Cruddas, the son of a Smithfield meat porter and CMC's executive chairman, had been set to underscore his reputation as the City's richest man by cashing in 30 per cent of his holding for around £240m.
Mr Cruddas had even prepared Deutsche Bank, JP Morgan Cazenove and Investec, who are running the listing, to sell a further 3 per cent of his stake should demand prove hungry.
They have spent the past week trying to drum up interest among major investors here and in America, both for Mr Cruddas' stake and for the new shares CMC wants to issue to raise £80m.
The reception among investors was said to have been warm but they are thought to have been unsettled by the drubbing financial companies' shares have taken in recent days.
Global stock markets have suffered steep losses in recent weeks amid concerns over sliding commodity prices and rising US interest rates.
Extreme market volatility has been good for business at CMC but may force the company to cut its float price or delay its listing.
Mr Cruddas founded CMC in 1989 with a £10,000 investment and has built the company into a global player. CMC specialises in trading contracts for difference, an increasingly popular financial instrument, for private investors.
It also offers spread betting, gold, oil and foreign exchange trading, with operations in the US, China, Australia and Germany.
Mr Cruddas, whose twin brother is a London cabbie, owns all the company save for minor stakes held by key employees. He has amassed an estimated £864m fortune.
Last year, the twice-married father of four paid himself £31m, setting himself among the very best paid of British businessmen.Reuse content