They may prefer to call it being 'economical with the actualité'. But it's official: politicians just can't help telling lies.

A new study of the art of telling political whoppers, from the cash-for-questions scandal to Bill Clinton's sex life, concludes what cynical voters have long suspected - that it is almost impossible for modern politicians to tell the truth, the whole truth, and nothing but the truth.

The catch? Apparently, it is mostly the electorate's fault for asking too many questions in the first place.

'Politics should be regarded as less like an exercise in producing truthful statements and more like a poker game,' said author Glen Newey, reader in politics at the University of Strathclyde. 'And there is an expectation by a poker player that you try to deceive them as part of the game.'

His study concludes that the drive towards an open and accountable modern democracy - forcing Ministers to answer ever more exhaustive questions on issues they used to be able to gloss over - creates a 'culture of suspicion and makes it more likely that politicians will resort to evasion and misrepresentation and lies'.

'When journalists or other parliamentary colleagues start to probe at that area which the Government wants to keep secret, you are likely to be pushed further and further towards the territory of lying,' he said.

'I don't think there's any good way around that problem. Politicians need to be more honest about lying.'

Lying techniques varied from blatant misrepresentation of the facts, to using highly ambiguous language - as when Clinton insisted he 'did not have sexual relations' with Monica Lewinsky. It later emerged he was not counting oral sex as 'sexual relations'.

Newey's report - published by the government-funded Economic and Social Research Council - adds that not only is lying 'sometimes justifiable' where there is a public interest, for instance where national security is at risk, but voters even have a 'right to be lied to' about things where they would rather not know what had happened, such as what was done during a war.

Clare Short's accusation last week, on resigning from Cabinet, that the Prime Minister had misled her over Iraq focused renewed interest in truthfulness at Westminster. MPs regularly complain that an unfair assumption that they are all liars is eroding public confidence in politics.

The problem is, psychologists say, that politicians who do lie are usually very good at it.

'Politicians are often "high self-monitors" - people who are very conscious of the image that they give off: they are able to say "if I hesitate over this, use that particular word or hold my hands like that, people will think this",' said Dr Richard Wiseman, a psychologist at the University of Hertfordshire.

Most get caught in the end however, Wiseman says, because keeping a consistent yarn together 'if you are leading a complex life, as most politicians do, is pretty hard'.