Michael Spencer, the Conservative Party treasurer, has admitted that without informing the stock market he pledged shares worth about £15 million in a company he chairs to a bank as security for a loan.

The announcement that a 12 per cent stake in Numis, the City stockbroker, is “included in a package of assets charged pursuant to a loan facility” taken out with HSBC in October comes after the disgrace of David Ross, the Carphone Warehouse co-founder. It emerged this month that Mr Ross had secured a loan of more than £100 million on his holdings in four public companies without registering the fact.

It is understood that the original agreement between Mr Spencer and the bank to take the shares as collateral dates from January and that all changes in the ownership of such share stakes should be formally announced through the London Stock Exchange. However, it is a grey area legally whether pledging the shares as collateral constitutes a change of ownership, a point made on Mr Ross’s behalf by his supporters.

There is further confusion because the rules applying to fully listed companies and to those, such as Numis, which are quoted on the Alternative Investment Market, are different. AIM company directors are not required to tell the company about such arrangements.

A formal announcement from Numis yesterday stated that more than 13 million of the company’s shares owned by two private investment vehicles were part of the security for the loan. Mr Spencer and his family control the two vehicles, IPGL and its subsidiary Icap Finance.

It is thought that after the David Ross affair, Numis went to each of its directors and asked them if they had made similar arrangements with their banks. Mr Spencer confirmed that he had and the disclosure was made.

Mr Spencer also owns more than 20 per cent of Icap, the interdealer broker that he founded. Part of this stake, also held by IPGL, has also been put up as collateral to raise funds to invest elsewhere. Such arrangements are common among entrepreneurs who do not want to sell the shares outright.Mr Spencer’s private interests also control City Index, the spread-betting company, and various leisure assets, including restaurants.

The formal announcement from Numis said that at the time of the loan IPGL took independent advice and believed that the arrangement did not constitute dealing in the shares.

However, “in the wake of recent press reports on this issue”, an appar- ent reference to the David Ross disclosures, Mr Spencer sought further advice “and concluded that notific- ation should now be made”. The AIM authorities have been tightening the rules of disclosure and trying to make them more transparent. “I think AIM’s advice on it has changed a little in the light of recent events,” an observer said.

Mr Spencer’s true fortune is his Icap stake, worth more than £350 million.

Interdealer brokers allow banks and other institutions to deal anonymously in bonds, derivatives and other exotic instruments. They do not take risks on their own part and have done well out of the market turmoil of the past year. Mr Spencer’s business volumes can exceed $1 trillion (£678 billion) on a good day and has about a 30 per cent market share. However, Icap’s shares have been slipping in recent months as analysts worry that the sector’s rapid growth is slackening.

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