When the CBI director-general advised the Chancellor to respond to current economic turbulence by delivering a six-paragraph, do-nothing budget speech this Wednesday, some legacy items from Gordon Brown's final budget last March were clearly not up for renegotiation.

One of Brown's final acts as Chancellor was to cut the main rate of UK corporation tax by 2p, to 28p in the pound. That change does not come into effect until next month. But the CBI doesn't want to see it scrapped until the near-term outlook becomes clearer.

No. The lobby group wants Alistair Darling to go an awful lot further. To restore the competitiveness of UK-based business, it claims, the tax rate on corporate profits here needs to be slashed back - to a rate of just 18%, over the next eight years.

It does want the scheduled increase in the rate paid by SMEs - due to rise under Brown's 2007 changes, from 20% to 22%, from next month - to be scrapped and reduced instead - to that same 18% figure over a three-year period.

And if the CBI had its way, the North Sea oil and gas industry would see the tax it pays on its profits cut from 50% to 40%.

It is, argues director-general Richard Lambert, all about bringing in a corporate tax environment which allows Britain to go on competing in a globalising world, where big companies, the main corporate contributors to government coffers, are increasingly mobile and willing to exploit differential tax rates, playing country against country.

"Threats firms make to move their headquarters away from the UK are in no way empty," he warns. "The government ignores them at its peril."

This radical tax-cutting package is light years away from what even the Cameron-led Tories are offering. They want to get the main rate down to 25%, to be paid for by scrapping a lot of current allowances. Lower headline rate, same revenues. The fiscal equivalent of rearranging deck chairs on the Titanic The CBI isn't having any of that. Its plan has been drawn up by a task force, made up of senior executives from an array of multinationals who pay the tax. Its chairman, Charles Alexander, is president of GE Capital Europe. Its members include senior figures from BP, Barclays, Cadbury Schweppes, Pfizer, Fidelity and Rolls-Royce.

They make it plain this isn't just a catch-up exercise, designed to match the growing tax challenge from established competitors, like Spain, the Netherlands and Germany, and from some newer rivals from the European Union's eastern rim.

Rather this is a road-map which, if implemented, would see the UK not just match the current Organisation for Economic Co-operation and Development (OECD) and EU averages, but undercut them well into the medium term.

In 2005, the average tax rate on company profits across the EU was still 26.1%. Across all of the OECD countries it was 28.8%. So an 18% UK rate would buy quite a lot of breathing space. But at some considerable cost in lost Treasury revenues, even on the CBI's own calculations.

It puts the raw cost, in corporation tax revenues foregone, at £11bn by year four, £20bn by year eight (when the 18% target rate is reached) and £27bn by year twelve.

But, of course, the case for corporate tax cuts is never put in raw terms.

Proponents always argue that lower tax liabilities lead, inevitably, to higher investment, improved productivity thanks to the resultant increase in capital stock, improved wages and, in the end, higher GDP growth.

However, even on this, more "dynamic" analysis, the CBI task force still projects a fall in government tax revenues in the first five years of cutting rates - as much as £4.2bn in year three, falling to £300m in year five, before the good times start rolling in.

By year eight, tax revenues from the boost to national growth have recovered sufficiently to add an extra £5.3bn to Exchequer revenues. But year 12 it's a soaraway £26.2bn more. But on one tricky matter the CBI analysis is silent.

How is any government supposed to present its voting public with years - five at least, even on this study's more dynamic assumptions - of reduced tax revenues and, presumably, cuts to services, and still hope to stay in power?

And there is another rather pressing question about the possible consequences of the CBI's corporate tax road map for the UK. Its task force tells us a 10% average effective corporation tax rate has been found to increase inward investment by US multinationals by 60%.

The UK, despite its deteriorating competitive tax position, remains a significant beneficiary of foreign direct investment(FDI), especially from US multinationals. In 1997, it captured a remarkable 31% of all FDI coming into Europe. By 2005 that had fallen to 18%.

But if UK corporate tax rates were slashed to 18% over an eight-year period and rival economies retaliated, as they surely would, just what share of FDI might the UK capture then? Who knows. What chance it would be that whopping 60% increase pencilled in by the CBI? Fat chance, I suspect.

Somehow I doubt Darling will be rushing to embrace this particular CBI package tomorrow.