A group of 24 peers have been have been placed under investigation following allegations they breached transparency rules by failing to give details about the companies they run.

       Conservative peer Lord Julian Fellowes, creator of the Downton Abbey series, is among the group of politicians to be probed by a parliamentary watchdog over their financial interests.

       Cross-bencher Lord Alan Sugar, star of The Apprentice, will also be investigated by the Lords Commissioners for Standards – the body examining whether peers failed to properly declare details about their companies.

       Campaign group Unlock Democracy had submitted a formal complaint to the watchdog – claiming dozens of peers were in breach of the rules by failing to provide “clarity” about the nature of their businesses.

       Tom Brake, the former Lib Dem MP who is now director of Unlock Democracy, said: “I welcome the fact that the Lords Commissioner for Standards has launched an investigation into a large number of peers.”

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       The campaigner added: “We are making a simple request; that peers are upfront about their business dealings. Without this transparency, people cannot judge whether peers are speaking in the public’s interest or their own.”

       It follows an initial investigation by Open Democracy which highlighted the potential breach of transparency rules over the apparent failure to declare basic details about the private companies they run.

       Code of conduct rules state clearly that if a peer is a director of a company, they are expected to explain to the parliamentary what that company does “where this is not self-evident from its name”.

       Major Conservative Party donor and Tory peer Lord Bamford – owner of the digger firm JCB – is another of the 24 peers subjected to the probe after details were announced on Monday.

       His declaration on the Lords’ register of interests lists his directorship in a company called Editallied Limited, but does not provide any more details.

       Labour peer Lord Carter is also under investigation by the standards watchdog. He has declared his directorship of offshore company Primary Group Limited, based in the tax haven of Bermuda – but has not provided further details.

       Tory peer Lord Nat Wei claimed he had made only an “administrative” error in his declaration, saying he had already apologised to the commissioner and updated his details.

       “I believe the omission of information on my register of interests was an administrative oversight on my part and having apologised for this to the Standards Commissioner my register of interests has now been updated in line with the revised code of conduct,” he told Business Insider.

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       Labour MP Margaret Hodge, the former chair of the Public Accounts Committee, said in July when the potential rule breach emerged that “the whole thing leaves a bad taste in the mouth”.

       She added: “Failure to [explain what a company does] is of course not itself an indicator of wrongdoing, but the sheer scale of the problem shows that there is a problematic lack of transparency in the Lords”.