Britain's banks must show regulators on Tuesday they could cope with two emergency scenarios -- a big cyber attack stopping customers withdrawing cash, and public transport disruptions during next summer's Olympic Games.

Barclays, HSBC, Lloyds, and Royal Bank of Scotland are among 87 financial firms taking part in a market-wide exercise to check how quickly they can return to 'business as usual' after the simultaneous scenarios.

There are no 'passes' or 'fails'. The exercise is about firms assessing their business continuity systems and updating them where necessary and the authorities identifying areas for further attention, the Financial Services Authority said.

The FSA is coordinating an exercise that kicks off at 8 a.m. and was set to be completed around late afternoon. The Bank of England and the finance ministry helped put together the two test scenarios.

The exercise is voluntary and one of the largest of its kind in the world, involving thousands of staff.

While Britain has been looking at contingency plans in case the euro zone breaks up, this won't form part of Tuesday's test.

The watchdog will publish a summary of its findings in February but any firm that struggles to show a convincing response will have questions to answer.

The scenario for a cyber attack will involve the internet crashing, along with telecoms and wholesale and retail payment systems such as high street bank ATMs, or cash machines.

The FSA will be checking how firms tell customers who can no longer use their credit cards, make payments, take out cash or check accounts online.

The response would be set against a backdrop of severe travel disruption during next year's Olympics when the number of travellers is set to swell.

The combination will look at what firms do when they cannot contact employees or if staff cannot get to the office to deal with a systems and communications crash.

Firms will also have to look at what happens if they cannot take delivery of essential goods and services.

(Reporting by Huw Jones; Editing by Dan Lalor)