Four British politicians have told a court they are not guilty of charges of false accounting over their parliamentary expenses, an issue that has tarnished the image of the country's major parties with an election only weeks away.

The case, which opened at Westminster magistrates' court in London, is the first criminal prosecution stemming from a wider scandal that erupted last year over politicians' expenses.

The four men involved are Elliot Morley, David Chaytor and Jim Devine, all Labour members of the lower parliament, and Paul White, known as Lord Hanningfield, a Conservative member of the upper house.

Police began investigating the matter after details of all MPs' expenses claims were leaked to the Daily Telegraph, a British newspaper.

Privilege argument

The expenses scandal dominated newspapers' front pages for weeks, as it emerged that many members had claimed a variety of expenses, including toilet paper, a shelter for ducks and the cost of cleaning a moat.

The ruling Labour Party has barred Morley, a former minister, Chaytor and Devine from running for new terms in the upcoming election.

White has resigned as the Conservative Party's business spokesman in the upper chamber and been suspended as a member of the party's group in the assembly. 

The lawyer representing Morley, Chaytor and Devine argued before the court that parliamentary privilege should prevent them from having to face a criminal trial.

Julian Knowles said: "They maintain that to prosecute them in the criminal courts for their parliamentary activities would infringe the principle of the separation of powers, which is one of the principles which underpins the UK's constitutional structure." 

After a brief appearance at the court, a short distance from parliament, all four men were released on unconditional bail and ordered to appear before a higher court on March 30.

Britain is expected to hold a general election on May 6 and the latest polls show the result could be inconclusive, with neither of the two main parties winning an outright majority.

The expenses scandal makes the election outcome even harder to call, with smaller fringe parties and independent candidates expected to pick up votes from disgruntled Britons.

The four men face a maximum sentence of seven years in jail if convicted.