Among the more obscure provisions of the last Labour manifesto was a commitment to undertake a review of the various voting systems introduced since the party came to office. That exercise was duly undertaken at the old Department for Constitutional Affairs, completed months ago but remains unpublished. A copy has, though, come into the hands of this newspaper, as we report today. It concludes that many of the arguments made in favour of electoral reform for the Westminster Parliament are spurious.

This is, by implication, of wider embarrassment to its advocates. Britain (excluding Ulster) once had the most straightforward of voting arrangements. The simple plurality or first-past-the-post system was employed for all contests. Since 1997, however, an array of new methods has been introduced at many levels. Different versions of the Additional Member System (AMS) have been imposed for elections to the Scottish Parliament, the Welsh Assembly and the Greater London Assembly. The Single Transferable Vote (STV) was selected for the Northern Ireland Assembly and also most recently for elections to local councils in Scotland. Mayors in London and elsewhere are selected by the supplementary vote (SV). Many in the Labour Party have suggested that the Alternative Vote might be valuable if the House of Lords is reconstituted. And a party list is the means by which members of the European Parliament are chosen.

If these changes have added anything to British politics, it is confusion. This was dramatically illustrated in Scotland earlier this year when the decision to hold an election for the Holyrood Parliament by AMS and local government seats through STV meant that many ballot papers were incorrectly handled by voters and the credibility of the entire electoral process was brought into question. It is doubtful whether Londoners who will vote for their mayor by SV and their Assembly members by AMS next year can explain how their preferences are to be translated.

This might not matter if the virtues claimed for proportional representation occurred. The evidence, though, is that they do not. There is no reason to think that people will be more likely to vote if they calculate that a new electoral system will somehow make their voice louder in the democratic chorus. And there is no proof that a different electoral system would lead to more women or people from ethnic minorities in the House of Commons. All that would be ensured is a far higher prospect of there being no overall majority for any party .

This is the nub of the issue. The current electoral system has the drawback of giving the largest single minority at Westminster an extremely large share of political power. Yet proportional representation would mean that much smaller minorities would wield undue influence, as without them it would not be possible to form a stable administration. Would this constitute progress?

It is not surprising, therefore, that the official Review of Voting Systems could not work up any enthusiasm for overhauling the current system. What is more intriguing is why the 110-page report has not seen the light of day until this morning. There has to be the suspicion that Labour, aware that at some point it might need the assistance of the Liberal Democrats to survive in office, is unwilling to offend its potential partners by publishing a document which is so damning of their pet project. Sustaining a dubious deal at a later date is surely the worst argument for PR.