Police officers are taught that an informant’s identity is sacrosanct. This is an iron law: detectives would rather collapse a court case than reveal the identity of a confidential source. This means the Metropolitan Police’s blunder in the Peter Mandelson case is significant. Officers this week clumsily identified Lindsay Hoyle, the Speaker of the House of Commons, as the source of information that Mandelson presented a flight risk. Hoyle’s tip-off led to the disgraced Labour grandee’s surprise arrest on Monday, contrary to an agreement to interview him by appointment. To make matters worse, officers leaked the Speaker’s identity directly to Mandelson’s defence lawyers — Mishcon de Reya, one of London’s most ruthlessly efficient legal firms.

While mistakes happen, this affair augurs badly for the Met’s Specialist Enquiry Team, the New Scotland Yard unit entrusted with delicate investigations. Precisely how Hoyle’s details were leaked has yet to be revealed, although there’s a suggestion it was via custody documents disclosed during Mandelson’s detention at Wandsworth Police Station. In my entire career, I have never seen the identity of an informant entered onto a custody record or any other material likely to be disclosed to a defence solicitor. This is deeply embarrassing for the Met in general and the Senior Investigating Officer in particular.

The very word “informant” conjures visions of seedy “grasses”, but in policing, it simply refers to anyone offering information with an expectation of confidentiality. And a case like Mandelson’s is a relatively genteel affair by the standards of the average criminal investigation. Yet iron laws are iron for a reason — and if detectives can make errors in high-stakes cases such as this, then they can make them in others, too. Breaching such basic principles hardly speaks to competence, and it may well lead to others choosing not to share information.

How then could such a mistake occur? From the information currently available, it would appear that basic due diligence failed during the custody procedure. Contrary to the conventions of TV drama, senior detectives hardly ever make arrests, which is left to detective constables and sergeants. Normally, an SIO will send their most competent and experienced officers to make an arrest in these circumstances.

What’s more, there was no good reason they even needed to know the identity of the source who claimed Mandelson might abscond abroad. Firewalls should have protected any confidential information received by the enquiry team, but in this case, it appears more than a few officers were aware of the source’s true identity. This is, to be blunt, inexcusable. It is also indicative of how specialist CID units lost their intelligence management capability during another of the Met’s indeterminable cost-cutting exercises.

All this points to the ongoing degradation of the Met’s detective branch. Specialist squads are only as good as their detective constables and sergeants. Cuts to policing mean CID is now a performance-driven stat factory, with investigators chained to their desks. Indeed, many view it as a punishment posting. Officers can now join up as direct-entry detectives, meaning trainee investigators lack vital street policing skills, such as informant confidentiality. Add to this a reduction in specialist units, low morale and an inexperienced workforce, and it’s no wonder the Met faces a challenge scouting for talent. Against the likes of Mishcon de Reya, it doesn’t have a chance.