The Inland Revenue is behind the first change, which requires firms making profit of more than £1.5m to pay tax in instalments for the first time. Accountants say the change will have serious cash flow implications for firms that fail to gear up for it.
Directors of listed firms must also carry out an assessment of the way they handle risk under a corporate governance code that came into effect on 1 January.
The new Combined Code is an amalgam of the Cadbury, Greenbury and Hampel committee reports.
Mark Watson, corporate governance executive at the Institute of Directors, said directors will have to take a much harder look at the risks a company faces.
Directors must provide a full description of how they look at the risks to satisfy auditors and shareholders. These include health and safety issues, financial procedures, environmental risks and regulatory compliance.
Watson said: "In essence, directors should be doing this already. The code means they have to comment on what they are doing in detail in the annual report for the first time." He said the code brings directors' duties to the fore for the first time, with firms likely to face a tougher grilling at annual general meetings. Mowlem finance director Gerry Brown said the code is a heavy compliance burden for contractors in a high-risk industry.
But he welcomed the idea that businesses would be more transparent. He said: "It is a good chance to put across the message that the industry is putting its house in order." The Inland Revenue's corporation tax self-assessment scheme means firms must pay tax more promptly in instalments instead of in a one-off payment nine months after their financial year-end.
Carol Barrie, head of tax in the Midlands division of chartered accountant Robson Rhodes, said the short-term effect of the change will be that firms will face a tax double-up in the next financial year.
She said: "Although the scheme is being introduced gradually, it still means firms are going to be hit with their old tax bills and new assessments in the same year.
"A lot of firms have not perceived quite what an impact this will have on cash flow. If they do not budget, it could come swinging at them and hit them between the eyes." Barrie said the change also means that tax inspectors could demand access to accounts for the first time.