It is understood the group has warned the Metropolitian Police that probes into expenses may breach “­parliamentary privilege”, the ancient right that protects proceedings in Parliament, which notably allows freedom of speech in the Commons chamber.

A senior Labour source confirmed some MPs have taken legal advice on whether parliamentary privilege extends to expenses, potentially putting them off-limits to the police.

Although it was reported yesterday that Labour MP Elliot Morley may be charged in the New Year, detectives fear they may not be able to make arrests even if a politician has made fraudulent claims.

The arrest of Tory immigration spokesman Damian Green at the Commons last year has left the Met ultra-sensitive to any criticism of how it deals with parliament.

The revelations come after Met officers visited Scotland last week to interview witnesses as part of their probe into Labour MP Jim Devine.

Labour banned the member for Livingston from standing as a candidate at the next election amid concerns over his expenses.

The MP’s £2157 claim for getting his London flat re-wired aroused suspicions after it emerged that the invoice supporting the work was from a firm that does not exist.

The invoice contained a bogus VAT number and a sham postcode. At the time Devine said tradesmen had provided him with the invoice that had been submitted.

The former union official was also asked to explain why the taxpayer was charged £2326 for shelving and joinery work done by his local publican, Tony Moran, in the West Lothian village of Blackburn.

With no shelving on the walls of his constituency office, Devine said the items were in a pub cellar he had rented. The Met subsequently launched an inquiry following a complaint by the Sunlight Centre for Open Politics.

The Sunday Herald understands at least four detectives travelled to Scotland last week. They are thought to have interviewed at least eight witnesses in the course of their inquiries. One of their visits was to the Crown Inn in Blackburn, the pub where Devine claimed the shelving was stored.

Although the cellar was demolished earlier this month, the new owner kept the rusting shelves.

The officers photographed the pub and the shelving and took a statement from the owner, as well inspecting the pub’s attic in a search for more shelves.

It is believed they have also interviewed Moran. Police are also expected to check Devine’s bank records.

Devine and two other MPs are being probed by the Met, while it was reported yesterday that HM Revenue and Customs is investigating the expenses of 27 MPs.

The Livingston MP claimed last week that Sir Thomas Legg’s audit of MPs’ expenses had cleared him of wrongdoing.

However, a spokesperson for Parliament said that Legg would not examine any claims currently under investigation by police. Legg’s remit also excludes office expenses – the budget used for the disputed shelving.

As well as the police inquiry, Devine is facing two employment tribunals by former staff. Two women, Marion Kinley and Linda Chatwin, have

hearings scheduled before the end of the year. It is also understood that Kinley has launched a defamation action against the MP.

A separate probe by the Parliamentary Commissioner for Standards into Devine’s travel expenses has been halted due to the police investigation.

A spokeswoman for the Metropolitan Police, asked whether the force believed it would not be able to arrest MPs, said: “It’s not something we would discuss. That’s obviously speculation.”

A Labour Party spokesman said: “We always expect all Labour Party representatives and members to co-operate fully with the police.

“The Labour Party has already taken tough action on expenses, including barring MPs found to have broken the rules from standing as Labour candidates at the next General Election.”