The warning comes from the technology watchdog Action 2000 which says property owners and managers must ensure their systems are fully compliant.

With barely six months to go before the millennium there are increasing fears about the effect on ordinary day-to-day systems which are reliant on computerised dates.

Action 2000, the government-backed body which is trying to ensure the UK does not "crash" on 1 January 2000, launches a campaign on Sunday designed to further raise awareness.

It says lifts, escalators, close circuit cameras, heating and air conditioning systems and fire alarms could all experience gremlins at the dawning of the new millennium.

A guide and checklist have been published in the hope of making landlords aware of their obligations.

'Safety implications'

Action 2000 chairman Don Cruickshank said: "It is the responsibility of developers and building managers to ensure buildings and their occupants are safe.

"There is an urgent need for proper investigation because resulting disruption to business operations could have wide-ranging safety, commercial, financial and legal consequences."

One of the biggest worries is security entry systems, which are often controlled by a computer.

Thousands of tenants could find themselves locked out - or locked in - on Millennium Day and similar problems could affect those using rented offices.

'Sue for loss of earnings'

Mr Cruickshank said: "Tenants of buildings could hold landlords or premises managers negligent and sue for loss of earnings from not being able to work from their offices."

If companies cannot enter buildings "owners could be sued for millions and held responsible for any loss of earnings for that company", warned an Action 2000 spokesman.

The millennium bug headache is just one of the problems facing firms on 31 December and 1 January.

Many also face the prospect of offering expensive "sweeteners" to nightwatchmen, cleaners and IT support staff to tempt them into work on days which are both now Bank Holidays.