Technicians are working through the weekend to establish how many share accounts were involved and to fix the problem. But Halifax said thebreach is likely to have affected only a "very small handful" of people, and it does not expect to find that any unwanted transactions were made.
David Walkden, general manager of Halifax Direct, promised that if people had "made mischief", all unwanted deals would be reversed.
The problem originated after close of trading on Thursday when engineers tried to amend the dealing system, which was introduced last month. The loophole came to light shortly after dealing began at 8am yesterday, and the service was suspended at 10.15am.
Mr Walkden said people could have accessed other dealers' accounts in a "very specific set of circumstances", including the manner in which they logged on to the service and the way data passed between their account and Halifax's main database.
The Securities and Futures Authority said it would be watching the situation closely.Reuse content