The leaders of the Church of England have every right to pronounce on political issues. Like the other great monotheistic faiths, the Christian churches believe that faith must be lived out in the world and cannot be divorced from political debate.

Nevertheless, sincere though their intentions undoubtedly are, the content of the latest statement from the two archbishops of Canterbury and York, warning people not to vote for the far-right British National Party (BNP) in next month's European and local elections, must be counted a mistake.

Dr Rowan Williams and Dr John Sentamu are playing directly into the hands of the BNP by whipping up yet more brouhaha over what is likely to remain a marginal force in Britain's political life. Quite inadvertently, they are feeding the party with the oxygen of publicity to a degree that must be delighting the party's leader, Nick Griffin.

The two archbishops are not alone in pursuing this unwise, counterproductive course. Representatives of the mainstream political parties have been queuing up recently to urge people not to vote for the BNP, planting in many people's minds a connection between the scandal over MPs expenses and voting for the BNP that probably never existed before they first raised the matter.

It would not have occurred to most of the electorate to register their disapproval of the conduct of certain greedy Conservative, Labour or Liberal Democrat MPs by switching votes to a party whose only real, substantive issue is an immediate halt to immigration. The argument being put forward in recent days by various MPs – that we must all continue to vote for the traditional parties or risk handing over the country to fascists – comes across as unscrupulous and self-interested.

The fact is that unlike in France, Italy or Austria, the extreme right in Britain has never made a significant breakthrough. The BNP has developed into a significant political force in only a handful of areas, mainly in a few troubled towns in the north-west of England that have experienced prolonged high unemployment as a result of the collapse of traditional industries and which have been under the complacent management of a single political party for decades. Outside these pockets, the BNP has made few inroads. And it showed no sign of becoming the principal beneficiary of public anger against the big parties over the business of expenses until the politicians and archbishops decided to give them a great deal of airtime. If, as this newspaper reports today, the party is now registering an unprecedented number of hits on its website, that is surely, at least in part, the unintended outcome of all this free publicity it has been receiving.

The way to contain the BNP is not to rail in a frantic and exaggerated fashion against the great "threat" that it apparently poses to British democracy but, where and when it is appropriate, to calmly remind people that the arguments it proposes are wrong, illogical and unworkable – as well as obnoxious. Voters should be reminded that we cannot become the insular, isolated state for which the BNP pines, nor should we wish to.

What we should not do is what we are in danger of doing now: grossly exaggerating the BNP's size and importance and so, quite possibly, handing it a great many votes that it would not otherwise have hoped to have gained.