The Members of Parliament who are threatening legal proceedings because Sir Thomas Legg, QC, has asked them to pay back expenses are in danger of throwing good money after bad. The courts would give them almost as unsympathetic a response as they are likely to face from their constituents at the general election next May. No court would interfere with the internal workings of Parliament on these issues until the process has been concluded. And then a court would consider the final decisions to be within the discretion of the parliamentary authorities.

The Members Estimate Committee of the House of Commons appointed Sir Thomas to conduct an independent review of expenses claims made by MPs in the past five years. He was asked to examine payments “against the rules and standards in force at the time”, give members “a fair opportunity to make representations”, and then report his conclusions to the committee.

Last week Sir Thomas sent letters to MPs informing them of his provisional views that many should repay sums (in some cases substantial sums for gardening and cleaning expenses) and inviting their comments. He will consider their responses and then report his recommendations to the Estimate Committee.

If any MPs refuse to pay up, that committee may report the matter to the House of Commons, possibly via the Privileges Committee. If any member refuses to co-operate with a decision of the House, Section 13(5) of the Parliamentary Standards Act, enacted this summer, will give the Independent Parliamentary Standards Authority the power to deduct the overpayments from MPs’ future expenses claims — assuming that he or she is not, by then, a former MP spending more time in the garden, having retired or been replaced by an ungrateful electorate.

Last week Ann Widdecombe, MP, expressed a view shared by many of her colleagues: “There is a big question mark over the legality” of the approach adopted by Sir Thomas, she said, because he had “retrospectively changed the law” by adopting standards different to those in force at the time when the payments were made. She added that if any other employer behaved in this way by changing the rules on expenses, and then presenting employees with a bill, they “would be up before a tribunal”.

There is indeed a general legal presumption that rules may not be changed retrospectively to the detriment of those affected. The difficulty, however, is in the application of the principle, not least because the courts have said that the strength of the presumption depends on the unfairness of the conduct in question.

It is far from clear that Sir Thomas’s provisional conclusions do involve unfair retrospective changes to the rules. The problem with MPs’ expenses, and the reason for the appointment of Sir Thomas, is that the rules lacked clarity. He has imposed what he regards as proportionate limits on the amount that can properly be claimed. MPs may have a case for arguing that he has imposed limits that are too low. But they would find it very difficult to dispute that the “rules and standards in force at the time” implicitly required a test of proportionality. That the officials of the House of Commons failed to recognise those limits when approving fees does not affect the true meaning of the rules.

In any event, the Administrative Court would not entertain a judicial review application against the provisional conclusions reached by Sir Thomas. Judicial review is a potent remedy, as demonstrated last week when the Divisional Court decided to publish a summary of intelligence reports supplied to the British Government by the US (the publication has been put on hold pending an appeal). But judicial review is a remedy of last resort, to be exercised only when the decision-making process has been completed. There are many stages remaining before any MP is obliged to write a cheque. That some MPs have already chosen to do so, or have been forced to pay up by their party leader, is irrelevant.

That the decision-making process is within Parliament would make a court even less willing to intervene. When the 2009 Act was debated in the House of Lords in July, the Attorney-General confirmed that although decisions of the new statutory authority may be subject to judicial review, nothing in the legislation was intended to alter the well-established principle that parliamentary procedures should be exhausted before such litigation could be commenced.

MPs who believe that Sir Thomas’s provisional conclusions are a bit rich, and will unjustifiably make them poorer, would therefore be well advised to focus their attention on politics and not the law.

The author is a practising barrister at Blackstone Chambers, a Fellow of All Souls College, Oxford, and a crossbench peer in the House of Lords

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