A police investigation into the MPs expenses scandal will swiftly identify false accounting as the criminal offence most likely to have been committed by the most egregious of the SW1 claimants.

The offence is committed when a person dishonestly, with a view to gain, produces a record or document that he or she knows is misleading, false or deceptive in a material particular. An expenses claim constitutes a record or document produced for an accounting purpose and, if submitted dishonestly, the maker is liable to a maximum of seven years imprisonment on conviction.

A greater number of MPs ought to be worried because the offence can also be committed where a person knows his expenses claim may be misleading and likely to induce another person into believing that the claim was properly allowable. Proof that an expenses claim was misleading is not required. The purpose of the law is to defeat an argument that a claim was certain to mislead.

The twittering chorus of MPs asserting that their extravagant expenses and flipping contrivances fell within the rules presents a challenge to the legal system. The Green Book confers wide latitude on MPs to designate either their London or constituency accommodation as their main home. There is no guidance as to how frequently MPs may change this designation or sell their additional home and, if they sell, what is to happen to profits made on sale.

But it is fatuous for MPs to maintain that they have acted within the spirit of the rules. In 1995 the House of Commons adopted a Code of Conduct “based on concepts of selflessness, integrity, accountability, openness, honesty and leadership”. The Green Book states that “these broad principles . . . underpin the allowance regime . . . and . . . Members must ensure that claims do not . . . give the appearance of giving rise to an improper personal financial benefit to themselves or anyone else”. Parliamentary allowances “are designed to ensure that Members are reimbursed for costs properly incurred in the performance of their duties”.

Applying these criteria, claims for flipping, phantom mortgage repayments, swimming pool maintenance, tennis court repairs, moat clearance and the purchase of hanging baskets and pot plants (to mention a few) stretch credulity beyond its outermost limit.

Astonishingly, these claims were made even after Michael Trend, then an MP, was suspended from the House for two weeks in 2003 after claiming expenses on his Windsor home where he lived with his family. The Committee on Standards and Privileges ruled the MP had been wrong to treat his friend’s rent-free accommodation in London as his main residence under the allowance regime. The MP was also ordered to repay £90,000.

Police and prosecutors will be reflecting upon whether it is possible to establish a case of dishonesty against profligate MPs, notwithstanding that the public was deemed to consent to the claims through the approval of the House of Commons Department of Resources.

For centuries lawyers have debated whether a person who obtains a victim’s consent to suffering a loss (such as an elderly homeowner massively overcharged by a rogue decorator) is liable for theft where his conduct cannot be impugned in civil law. The present drift of the law suggests that it is no defence for a person to contend he is not guilty simply because he acted within the letter but outside the spirit of the law.

As Lord Steyn explained in a House of Lords decision, the purposes of the civil law and the criminal law are different and sometimes there is disharmony. When this occurs, it is wrong to assume that criminal and not civil law is defective.

The test of dishonesty applied by a jury in a criminal trial is two-fold. First, the jury asks whether the action was dishonest according to the ordinary standards of reasonable and honest people. If the answer is “yes”, the jury asks whether the defendant realised that reasonable and honest people would regard what he did as dishonest.

If a jury concludes that an MP had not realised reasonable and honest people would regard his action as dishonest, the verdict would stand as eloquent testimony of the remoteness of our lawmakers from the perception of the vast majority of law-abiding people.

In addition to challenging the boundaries of criminal law, the expenses scandal rekindles the debate about the relationship between the Attorney-General and the Director of Public Prosecutions. Although the Attorney-General, Baroness Scotland of Asthal, can recuse herself, the DPP is legally obliged to discharge his functions under her superintendence.

Although the suspects cross all parties and sit in the Commons, the Attorney-General remains a politician sitting in the Lords. As the police become more frequent visitors to Westminster, the need to change the constitutional arrangements becomes more urgent. Public confidence will be enhanced by ensuring the DPP is not subject to the superintendence of a politician in these circumstances.

The author is a practising barrister at 23 Essex Street and Visiting Professor (corporate and financial crime) at the London School of Economics

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