A similar move is being considered by Mori, but Gallup has ruled out the change at present on grounds of cost.
ICM says that research, which has included re-call interviews of those polled in the election campaign, has shown that if people had been handed a ballot paper to be marked, placed in a sealed envelope and handed back to the researcher, the average error in ICM's final poll for 9 April would have been cut from 2.3 per cent to 0.5 per cent.
The shift appears to have come not because people were directly lying to the pollsters but because the number refusing to answer was heavily cut when the ballot paper approach was used - down from 7 to 1 per cent. The number of 'don't knows' also fell 1 per cent to 4 per cent. Both groups proved to be predominantly Tory voters.
The shift, however, might be read to show that 'there's nowt so queer as folk'. For while voters appear increasingly reluctant to answer pollsters orally about voting intentions, the 'secret' ballot paper they hand back is simply unsealed later and the answer filled in on the questionnaire - thus preserving the carefully structured samples of age, gender and class on which opinion pollsters work.
Nick Sparrow, managing director of ICM, conceded yesterday: 'There is a danger that if it becomes widely known what we are doing, the refusals will go back up again.'
He argued, however, it was simply a question of making people feel comfortable about answering the question. 'We do sex surveys and things like that and people will answer about the most sensitive subjects if you allow them to do so in a manner where they don't feel embarassed.'
Mr Sparrow said the 'secret ballot' approach has been tried in the past for exit polls - where it again cut the refusal rate by 6 per cent, but from a much higher base - more than one in five refusing to co-operate in exit polls, against just over one in twenty for opinion polls. ICM says it will now use the method routinely for opinion polling and use any by-elections this Parliament to test out the technique.
Mr Sparrow said it appeared that 'the party preference question has become sensitive. People seem now to be more willing to answer when allowed to do so on an anonymous ballot paper in a sealed envelope'. Greater sensitivity might be the result of the breakdown of the old class structure, he suggested.
But while this time it was Tory voters who hid their intention behind a 'don't know' or refusal to answer, in the past - 1974 and 1983, for example - it appears that Labour voters did that.
In future ICM will also exclude people who say they definitely will not vote, while those who reject even the ballot approach will be asked which party has the best policies for the economy - a strong indicator of party preference, ICM believes.
A Gallup spokeswoman ruled out the ballot approach at present on grounds of cost - 'it's very labour intensive' - although Mori said it was considering it as one of a range of possible methodological changes.
Brian Gosschalk, director of Mori, said ICM had suffered a higher refusal rate than other polling organisations. 'We have to make a judgement on whether 1992 was an aberration or the new norm. When existing methodologies have served us pretty well in the past we are a bit wary of chucking out the baby with the bathwater.'