Michael Spencer, the City tycoon and Conservative Party treasurer, cut most of his financial ties with Numis yesterday as he announced plans to step down as chairman of the stockbroker and investment bank.

IPGL Limited, which is 55.1 per cent-owned by Mr Spencer, his wife and their children’s trusts, sold its entire 12.18 per cent stake in Numis. The shares were placed with institutional investors at 120p each, netting IPGL £15.7 million. The shares closed down 7¾p, or 5.7 per cent, at 123p.

Mr Spencer, best-known as the founder and chief executive of Icap, the interdealer broker, is retaining 20,000 shares in Numis in a personal capacity. In December he admitted that he had pledged Numis shares worth about £15 million as security for a loan without telling the stock market.

Numis, which is quoted on AIM, said that more than 13 million of its shares owned by two private investment vehicles were part of the security for the loan. Mr Spencer and his family own 55 per cent of the two vehicles.

In a stock exchange announcement, Mr Spencer said yesterday that he was stepping down because he wished to focus on his other business interests. He added: “It has been amazing to be part of Numis over the last five years, helping it grow from a firm with 62 people and 40 corporate clients to one of the leading independent investment banking and institutional stockbroking businesses that it is today. I wish the firm continued success in the future.”

Mr Spencer is one of Britain’s richest men, but he has been hit badly by the downturn, in which shares in Icap – in which he retains a 21.3 per cent stake – have fallen sharply. He recently said that he and his wife, Lorraine, with whom he has three children, were separating after 25 years of marriage.

Oliver Hemsley, chief executive of Numis, said: “We have been incredibly fortunate to have someone of Michael’s stature and calibre to help to guide the management team in developing the business and getting it into the right shape to serve the needs of our growing client base. He will be missed, but will remain a close friend of the firm.”

Last weekend, it was reported that Numis and Collins Stewart, which is chaired by Mr Spencer’s long-time rival Terry Smith, were vying to buy the corporate broker Hoare Govett, which Royal Bank of Scotland (RBS) acquired in last year’s takeover of ABN Amro, the Dutch bank. RBS is said to have told would-be buyers that the business was not for sale.

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