They estimate that over quarter of a million of Britain's 1.3 million small-to-medium sized firms are still not ready, despite repeated warnings over the last two years.

The millennium bug is a computer software problem which could result in systems not recognising the new date in the year 2000.

Mass mailing

Chairman Don Cruickshank said: "Action 2000 has a responsibility to ensure that every company in the UK is alerted to how to protect their business from risk of disruption from the bug.

Don Cruickshank: Last chance for businesses

"This is why we have taken the unprecedented step of mailing 1.3 million named senior individuals from those companies employing between one and 249 staff."

The booklet will be mailed to businesses from September 13 and is described as a "final lifeline". It targets areas "critical to survival" for companies which have not prepared and offers "short cuts" to emergency solutions.

Mr Cruickshank added: "Last Chance allows them to see at a glance the extent of their risk. And for those who are on track or have finished their programmes, it provides a final checklist to ensure they have covered everything."

Legal responsibilities

The guide, which is divided into eight sections, includes examples of the type of things that might go wrong, tips on legal implications, and useful contact numbers.

The total cost of the measures to tackle the bug in the UK is estimated at £25 billion with, for example, supermarket chain Sainsbury's spending an estimated £40 million.