The Metropolitan police have been accused of misleading behaviour in the phone-hacking scandal after handing over evidence they had twice claimed did not exist.

The latest embarrassment for Scotland Yard was disclosed in the high court in the case of Sienna Miller's stepmother, Kelly Hoppen, who claims that a News of the World journalist, Dan Evans, attempted to hack into her voicemail. The court heard she was a tabloid target because of her friendships with the former England footballer Sol Campbell and with Madonna's former husband, film director Guy Ritchie.

The case also threatens to embarrass the NoW because the alleged hacking occurred in June 2009 – three years after the arrest of its then royal correspondent, Clive Goodman, who was jailed with the paper's private investigator, Glenn Mulcaire, on the basis that he was the only journalist involved in intercepting mobile phone messages. News International, which owns the paper, has repeatedly said that it does not allow illegal news gathering.

Hoppen's barrister, David Sherbourne, told the court: "This case is enormously important because it drives a coach and horses through the claim that has been persisted in by News International and its executives, that the criminal activities of Goodman and Mulcaire were purely historic, the isolated actions of one rogue journalist and his private investigator associate."

The court heard that Hoppen suspected her voicemail was being intercepted in 2009 because private information was being published while some phone messages were recorded as "old" even though she had not listened to them. After the Guardian disclosed the scale of hacking at the NoW in July 2009, her lawyer, Mark Thomson of Atkins Thomson, wrote to Scotland Yard to ask if the evidence which they had gathered from Goodman and Mulcaire included any sign that Hoppen had been the object of unlawful interception. After three months, the Yard replied that they held no such evidence.

In April 2010, after receiving several alerts from Vodafone about unsuccessful attempts to access her messages, Hoppen's lawyer wrote again to Scotland Yard asking if Mulcaire had held her phone number or other personal details. After a delay of more than eight months, they finally replied in January of this year, once again claiming that they held no such evidence.

But last week, Sherbourne told the court, deputy assistant commissioner Sue Akers, who is leading a new investigation into the hacking, had contacted Hoppen to disclose that, in reality, the Metropolitan police had found hand-written notes which were kept by Muclaire detailing her phone numbers and two addresses, her mobile phone account number and the four-digit PIN code which was needed to access her voicemail. "This is the work of a professional hacker," said Sherbourne. Police had now handed over six different pages of Mulcaire's notes about Hoppen which they had been holding since they raided Mulcaire in August 2006.

Sherbourne said it was "regrettable, to put it mildly" that the police had twice denied that this material existed. "It could and should have been provided earlier," he said. "The simple and unavoidable fact is that they misled Ms Hoppen." Edwin Buckett, representing the Metropolitan police, told the court that police had acted in good faith. He said the material taken from Mulcaire was in a chaotic state and some of it was indecipherable.

In March 2010, her lawyers also had obtained a court order requiring Vodafone to hand over material relating to unsuccessful attempts to access her voicemail. This disclosed that on 22 June 2009 – the day after the Mail on Sunday claimed she was having a relationship with Guy Ritchie – her mobile had been called twice by a caller who witheld their own number; hung up the first call when Hoppen answered; then called back, got no answer and dialled into her voicemail for 25 seconds. Vodafone disclosed that the calls were made from a mobile phone registered to News International in the name of Dan Evans, a feature writer.

Mr Sherbourne told the court that in the summer of 2009, past and present News International executives had told a House of Commons select committee that they knew nothing of illegal activity by their reporters. He added: "It was at the very time that News International executives were giving this evidence to the select committee that Mr Evans made his attempts to access Ms Hoppen's voicemail using a News International telephone."

Michael Silverleaf QC, representing Dan Evans, said the evidence clearly showed that Dan Evans had dialled Hoppen accidentally. Evans remembered nothing of the calls. The keys on his phone were inclined to stick and to dial numbers accidentally. The use of his own phone to do something which he knew to be illegal would have been "quite unbelievably stupid". A search of his office and home computers had yielded no sign that he was interested in Hoppen until he was told of the allegation against him. The one occasion on which he appeared to have dialled into her voicemail was "one rogue call which nobody has yet explained", Silverdale told the court.

Mr Justice Eady made an order for Scotland Yard to disclose to Hoppen all relevant material, redacting only the detail of PIN codes. Hoppen has also applied for disclosure orders against Dan Evans and the NoW.