Branches of Barclays, Lloyds TSB, NatWest and HSBC have not linked up their IT systems for the scheme due to the size of the project and the data protection issues involved.
Data protection rules mean that bank staff are not allowed to see the balance of a customer from a rival bank.
The pilot service is being used by selected customers in 10 locations in England and Wales. When the trial is completed next January the banks will assess whether it is worth creating a common IT platform for the shared service, a spokesman for the British Bankers Association (BBA) said.
Financial services companies are eager to link their IT systems to improve customer services. Last year, some of the UK's leading mortgage providers linked their back-office IT systems via the Internet to enable financial advisers to offer customers provisional mortgage acceptances within minutes.
Legal & General, Abbey National and the Woolwich were among the companies that developed an XML-based standard for the system, which links financial advisers to an extranet and gives them access to a wide range of quotes.