So it is all nice and tidy is it? Mr Geoffrey Robinson, MP, Treasury minister, is not a tax avoider. Having been revealed by the Independent on Sunday as the beneficiary of a multi-million-pound offshore trust, Mr Robinson and two cabinet ministers stated authoritatively that all was in order. The arrangement with the Guernsey based trust did not involve any tax avoidance. Mr Alistair Darling, Chief Secretary, said that Mr Robinson had done what ministers with shares had done in the past and "put them into a blind trust".

It is easy to be dazzled and confused by such manoeuvres. Let us examine them for what they are. The question is why rich people such as Mr Robinson have trusts and put their assets into them. After all, to do so reduces, at least marginally, your freedom of action. Is not Earl Spencer claiming in the divorce court that his family's wealth is all locked up in trusts and therefore cannot be touched? And in terms of lawyers' fees and trustees' remuneration, trusts are costly to set up and maintain. One reason is to disguise the ownership of assets. Another is to minimise or avoid the inheritance tax which would otherwise be payable by the beneficiaries of your will; nowadays trusts are much less useful than formerly in avoiding income tax and capital gains tax. Here, then, is a straight question: will Mr Robinson's heirs enjoy tax advantages from his family trust in due course?

We may also ask ourselves why trusts are created offshore, in places like the Channel Islands or the Cayman Islands. The explanation is that putting a bit of distance between your assets and the Inland Revenue has its advantages. It may be, for instance, that the buying and selling of shares within the trust can be conducted without paying tax as you go along. In the United Kingdom, if I own ICI shares and sell them in order to buy BP, I shall have to pay tax on any gain in ICI even though I re-invest the proceeds in BP. Off-shore trusts can avoid that necessity. Tax becomes payable only when the beneficiary in this country receives a dividend or capital payment.

Furthermore, it is sometimes the case that tax payable becomes due rather more slowly when shares are held in offshore trusts than when they are held directly by a UK resident. Delay has a monetary value. A second question, therefore: are there tax advantages for the operation of Mr Robinson's trust as compared with owning the same assets directly?

If we cluck-cluck about Mr Robinson's offshore trust and wonder about the advantages it may confer, Mr Darling informs newspapers that the money was put into the trust by a Belgium woman living in Switzerland, so it was not a way of avoiding tax in Britain. Assets weren't taken out of the United Kingdom. Forgive me, but I think that you should always inquire into schemes which involve Belgium women living in Switzerland putting money into trusts based in the Channel Islands for beneficiaries living in Britain. Why not just send a cheque to Mr Robinson's home address or transfer the assets into his name? I can think of a possible reason why not. Because UK tax would become immediately payable on the benefit.

Equally curious is the ambiguity in Mr Robinson's statement over the weekend. On the one hand, he stated that he had never transferred capital or other assets into the trust - Mr Darling's point. That is on line 21 of his statement. He was just the fortunate beneficiary of Madame Joska Bourgeois' munificence. But then, on line 27, he also admitted that the "right" to buy pounds 9m of shares in the prosperous British company, TransTec, which he controls, had passed to the offshore trust. The minister states that capital gains tax is liable on the transaction.

When we look at the details of this particular transaction, we find a further oddity. Between Mr Robinson and the offshore trust was interposed another company owned by Mr Robinson, Stenbell. He sold his rights to Stenbell; in turn Stenbell sold them on to the offshore trust. Again there are two reasons why this complication might be thought advantageous: to disguise the ownership of assets; or to harvest a tax advantage. Was this the case?

Finally, there is the matter of the blind trust. Blind trusts are devices which place the management of your assets into the hands of trustees who do not, indeed may not, tell you what they are doing. The technique is designed to prevent conflicts of interest. Ministers say blind trust, end of argument. Actually blind trusts are not all they are cooked up to be. Ministers know that they will regain control of their assets when they leave office. We are never informed who the trustees are. For all we know they may be friends, associates, employees of the companies in which the minister has an interest, chosen because they know what to do without being told.

But whatever the merits of a blind trust, in this particular case Mr Robinson's evident conflict of interest has not been removed. The offshore trust established for the minister's family, of which he is a discretionary beneficiary, has not been included in the blind trust arrangements. This is a serious problem. Treasury ministers set the tax rules for residents of the UK who are beneficiaries of trusts, whether established overseas or not. Mr Robinson, as a Treasury minister, has a conflict of interest by virtue of being the beneficiary of a trust.

Nor can it be argued that Mr Robinson, as Paymaster General, has nothing to do with personal taxation. As it happens, this morning, Mr Robinson will present the Government's proposals for creating individual savings accounts that provide limited tax advantages for ordinary people, perhaps less generous than the Tory schemes they are designed to replace. The final question, thus, is for the Chancellor of the Exchequer: given Mr Robinson's conflict of interest, should he remove Mr Robinson from playing any role in the Treasury's management of the nation's tax system?