Twenty-four members of the House of Lords broke rules by failing to declare details of companies they run, it has been found.

The official ruling comes after an investigation by openDemocracy into one of the widest-scale breaches ever exposed.

The lords include Tory donor Lord Bamford, ‘The Apprentice’ star Alan Sugar, and the creator of Downton Abbey Julian Fellowes.

A report by the House of Lords Commissioner for Standards said the peers had “committed a minor and inadvertent breach of the Code of Conduct by failing to provide a description of companies they had registered”.

He added: “Each of them has resolved the breach to my satisfaction and I consider that no further sanction is necessary.”

But the breach was so wide-scale that the commissioner said there will now be an effort to make sure peers stick to the rules during the next annual audit of the Register of Interests. New entries in the register will also be checked before they are published.

Rules state that if a lord is a company director, they should “give a broad indication of the company’s business, where this is not self-evident from its name”. But openDemocracy found that dozens of peers had not done this.

Lord Bamford, who owns digger firm JCB and has given millions to the Conservative Party, said he was also the director of a company called Editallied Limited. But he had not given any further details about what the company did.

Likewise, Alan Sugar did not provide a description of one of his companies, Harper Fox Partners Ltd.

And Julian Fellowes had not given a description of his company, Praemium Intentus.

In total, openDemocracy identified 54 financial interests from 42 peers that may have been in breach of the rules.

Most of these peers were named in a formal complaint to the Standards Commissioner by Tom Brake, the former Liberal Democrat MP who is now director of Unlock Democracy.

“Our democracy will benefit when a clearer explanation of peers’ business interests is provided,” Brake said at the time. “This should already have been done.”

An official investigation was launched into 25 of them. The commissioner has now ruled against all of them – except the Duke of Wellington, who he said had provided details of his financial interests elsewhere.

Tommy Sheppard, the SNP's Constitutional Affairs spokesperson, previously told openDemocracy that peers’ failure to properly declare their financial interests highlights how “undemocratic” and “utterly unaccountable” the House of Lords is.

And Labour MP Margaret Hodge said the investigation “raises serious questions over the veracity of some entries in the register of interests in the Lords.

“It’s especially concerning to see major Tory party donors or close pals of the PM on this list,” she said. “The whole thing leaves a bad taste in the mouth.”