The Securities Commission today warned it may take action against large investors who don't make timely notification of alterations to their large stakes in listed companies.

The commission's warning comes as it said it would not take any action against Cushing family interests for their late filing of substantial shareholder (SSH) notices in relation to their stand in the market for Wakefield hospital shares last month.

Under securities rules, shareholders with at stake of 5 per cent or more in a listed company must immediately disclose to the company and the stock exchange any increase or decrease in their holding of more than 1 per cent.

Chairman Jane Diplock said disclosure was required "as soon as the person knows or ought to know" that their holding had changed by.

"This means that the market must be informed immediately the interest arises. It is not acceptable to delay this disclosure."

The commission had been looking at the issue following a stand in the market for 10 per cent of Wakefield Hospital shares on May 6. It later emerged the bidders were Sir Selwyn Cushing's H&G Ltd and Royston Hospital Ltd , "but the market was unaware of their identities until after the stand had gone unconditional and the market had closed on Monday May 9" the commission said.

H&G majority shareholders Sir Selwyn and David Cushing already held 7.37 per cent of Wakefield Hospital.

As the stand progressed, a substantial shareholder notice was due from 10.31am on May 9 but the Cushings, H & G and Royston did not file notices until after the market closed that day.

While the commission said the delay was not acceptable it believed there was no intention to withhold information from the market and would not take any further action on the matter.

" However it will continue to focus on timely compliance with disclosure obligations and will take appropriate action where it sees any failure to immediately disclose changes in substantial holdings."