Boris Johnson’s former ethics adviser has apologised for breaching House of Lords rules by helping a US satellite company influence Ministry of Defence (MoD) officials.

The Lords Commissioner for Standards found Lord Geidt, the late Queen’s former private secretary, broke the rules by giving some introductory remarks at a meeting in May 2021 between MoD officials and Theia Group Inc, which at the time employed him as an adviser.

Theia Group had put forward a proposal to enhance the UK’s space capabilities, and Lord Geidt told the commissioner his role had been to ensure the company’s contacts with the Government “were conducted properly”.

He also stressed that his role with Theia Group had been secured due to his prior experience and not due to his membership of the Lords.

But the commissioner found that his actions had contravened a prohibition on peers providing “parliamentary services” in return for payment, saying his attendance at the meeting counted as “assisting an outside organisation in influencing officials”.

Lord Geidt, who served as Boris Johnson’s ethics adviser between April 2021 and June 2022, had appealed against the commissioner’s findings, but his appeal was dismissed by the Lords Conduct Committee in a report published on Wednesday.

While the committee accepted that he had “sought at all times to behave honourably and to comply with the rules”, it found that he had provided a “parliamentary service” in return for payment and therefore broke the rules.

A source familiar with the matter said they were “mystified” by the decision, adding: “The lack of clarity about what constitutes parliamentary services is concerning.

“We note from the report that the House has since changed these rules, so that the same conduct now may benefit from an exemption.”

The committee described Lord Geidt’s breach of the rules as “an oversight” and limited to a one-off meeting in which he played “a very limited role”. It also noted that he had sought the advice of the Cabinet Secretary on his work with Theia Group, although this advice had covered only Government ethics rules and not the House of Lords code of conduct.

It upheld the standards commissioner’s recommendation that Lord Geidt write a letter of apology to the chair of the Conduct Committee – the lightest sanction available.

A spokesperson for Lord Geidt said: “The report acknowledges that Lord Geidt acted honourably in his evident desire to comply with the rules at all times.

“Despite that, the commissioner recommended the mildest sanction of writing a letter of apology to the committee’s chair for the ‘oversight’, which he has duly done.

“Lord Geidt has a formidable record of public service, including as private secretary to Queen Elizabeth and as the independent adviser on ministers’ interests.”

On Tuesday, the Conduct Committee also found Lord Mann had committed two breaches of the rules relating to his work as the Government’s independent adviser on antisemitism.

The committee supported the standards commissioner’s findings that Lord Mann had failed to record support from the Antisemitism Policy Trust in his register of interests and had used his parliamentary office for his work as the Government’s adviser.

On his appointment by the Government, Lord Mann had not been provided with an office or civil service support, but while the commissioner expressed “sympathy”, he found the peer had broken a rule requiring parliamentary offices be used primarily to support parliamentary work.

The Conduct Committee also expressed sympathy, saying there had been “an assumption in Government” that the House of Lords would provide accommodation needed to support his work.

On that basis, the committee downgraded the sanction recommended by the commissioner, requiring Lord Mann only to write a letter of apology to the chair of the committee rather than make a personal statement of apology in the House.