Brian Basham told the civil trial into alleged phone hacking of the Duke of Sussex and other figures that he warned David Grigson about the activities.

He claimed he told Mr Grigson to "clear the decks of all taint", after looking at MGN for a possible investment.

MGN denies senior executives knew about the practices and failed to stop them.

It is alleged that journalists from the newspapers obtained private and confidential information about people's lives through a variety of unlawful means between 1991 and 2011 - including accessing voicemail messages on their phones.

Mr Basham, a former journalist who ran a business that researched companies, was called to give evidence during the second week of a High Court case brought against MGN.

The court heard the entrepreneur had spoken to three industry insiders about phone hacking allegations at Trinity Mirror whilst researching whether to invest in the company.

"David did not heed my advice, which I believe was a big mistake. He behaved badly, the board behaved badly. He covered up and people suffered for years," he told the court.

"This company covered up wrongdoing."

When speaking to "journalistic friends", he said giving evidence: "It was like turning on a tap as soon as I spoke to them about this, it all poured out."

The group's legal director, Paul Vickers, was described to him as the "villain of the piece" because he was involved in the cover-up", which chief executive Sly Bailey "orchestrated".

He also claimed he was told the publisher's legal director Marcus Partington once joked that he had left his mobile phone at home but could ask a journalist if there were any messages, because phone hacking was rife.

Mr Basham claimed he detailed his findings at lunch with company chairman Mr Grigson in 2012.

During cross-examination, Andrew Green KC, on behalf of MGN, suggested Mr Basham had based his view of the company on unnamed informants.

"You were defaming a wide number of people to Mr Grigson in the hope he would pay you money to sort out the problem," Mr Green suggested.

"That is entirely untrue," Mr Basham said.

Prince Harry is expected to give evidence at the trial in June. He is among four people whose claims are being heard in the trial as "representative" cases of the types of allegations facing the publisher. They will also help the court set the level of damages MGN should pay if the claimants win.

Others involved are Coronation Street actors Nikki Sanderson and Michael Turner, known by his stage name Michael Le Vell, and comedian Paul Whitehouse's ex-wife Fiona Wightman.