These messages, sent in November 2020, were not disclosed to Lord Geidt when he exonerated Mr Johnson of wrongdoing in an earlier inquiry and called into question the Prime Minister’s claim he “knew nothing” of the payments until February last year.

While accepting Mr Johnson’s apology, Lord Geidt accused officials of failing to keep him informed and made clear that he believed his powers should be expanded to restore public confidence.

On Tuesday, Lord Geidt appeared to elaborate further, signalling in a letter sent to the Public Administration and Constitutional Affairs Committee that he expected these changes to include being given the autonomy to launch investigations into alleged rule breaches by ministers.

Currently, the independent adviser is required to seek the authorisation of the Prime Minister before launching a probe.

However, sleaze campaigners argue that this amounts to the Prime Minister “marking his own homework”, with the Committee on Standards in Public Life (CSPL) recommending that this requirement should be removed to give the adviser true independence.

Asked by William Wragg, the chair of the committee, whether he agreed with the proposal, Lord Geidt wrote: “As I wrote in my letter of 23 December, I would expect by the time of my next annual report in April to be able to describe the role of independent adviser in terms of considerably greater authority, independence and effect, consistent with the ambitions for the office that the Prime Minister has set out.”

‘Role not taken seriously’

It came as Lord Evans of Weadale, the chairman of the CSPL, questioned whether the failure by the Prime Minister to hand over the messages to Lord Geidt had been deliberate.

Asked about the controversy during a Parliament committee hearing, he told MPs: “I think it is absolutely transparent and crystal clear from the exchange of letters last week that the independent adviser did not feel that he had been well served by those people who had been providing him with information.

“And whether that was deliberate or whether it was careless, people can make their own judgment, and no doubt do.

“But it is absolutely clear that the independent adviser feels the role has not been taken as seriously as it needs to be, and I expect in the light of that exchange of letters that it will be taken more seriously in future, and I think that is a positive step.”