An appeal for information has been sent to all MI5 staff over false evidence the service gave to three courts in a neo-Nazi spy case.

MI5’s regulator is investigating how it came to make false claims about conversations with the BBC.

The Investigatory Powers Commissioner Sir Brian Leveson, who is overseeing the inquiry, said it was “surprising” that new material was being uncovered years after the case began.

Prime Minister Sir Keir Starmer ordered the investigation in September after the BBC revealed that MI5 had lied to three courts.

The prime minister’s demand followed requests from the High Court and Investigatory Powers Tribunal, which both rejected MI5’s explanations about what happened.

MI5 also gave false evidence to its regulator, Sir Brian Leveson himself, whose office is conducting this new investigation.

The case centres on a neo-Nazi state informant known as Agent X, who used his MI5 role to coerce his girlfriend, whom he attacked with a machete.

Sir Brian has now sent a letter updating the prime minister, the attorney general and the home secretary on the investigation into the false evidence.

In the letter, he reveals the investigation team “asked that a message be disseminated to all current MI5 staff”, inviting anyone who believes they have information which may assist, to make contact.

He adds: “I am grateful for MI5’s prompt and constructive engagement to achieve this.”

But he also notes that “relevant material” continues to be identified, adding that it is “surprising” four years after the case began and “over three years after my inspectors first considered the Agent X case.”

Earlier this month, MI5 was heavily criticised by the police investigation into the state agent Stakeknife for only disclosing crucial documents years after that inquiry began.

The investigation into the false evidence in the Agent X case is being led by Sir John Goldring, the Deputy Investigatory Powers Commissioner.

In the update letter, it is revealed that Sir John and his investigators have interviewed 36 people over the past five weeks, including “current and former staff of the intelligence services”, as well as BBC staff, government lawyers, and barristers representing MI5.

The new investigation is said to have identified “additional lines of inquiry” after obtaining material beyond that which was disclosed in the three court cases. The investigation report is not expected to be completed before March.

In February this year, the BBC revealed that MI5 had lied to three courts while defending its handling of the misogynistic MI5 agent, whom the BBC had sought to expose in a 2022 investigation.

Arguing for secrecy, the Security Service told judges it had stuck to its policy of not confirming or denying informants’ identities.

In fact, MI5 had disclosed Agent X’s status in phone calls to me, as it tried to persuade me not to investigate him. The service aggressively maintained its position until I produced evidence proving it was untrue, including a recording of one of the calls.

Following the BBC’s revelations, MI5’s Director General Sir Ken McCallum issued an “unreserved apology” for the false evidence.

Two official inquiries then took place which absolved MI5 and its officers of deliberate wrongdoing, claiming the false evidence was down to mistakes and poor memories.

But, in July, a panel of senior high court judges ruled that the “investigations carried out by MI5 to date suffer from serious procedural deficiencies” and that “we cannot rely on their conclusions”.

They said it would be “premature” to decide whether to begin contempt of court proceedings against any MI5 officers before a new investigation took place.

Sir Ken previously said the service would co-operate fully with the new inquiry.