The UK business minister, Kemi Badenoch, has written to Fujitsu to demand talks on how much it would pay towards compensating Post Office Horizon victims, after the scandal wiped more than $1bn off its value. The share slump followed testimony from one of its senior executives on Tuesday, who told MPs the Japanese technology company had a “moral obligation” to contribute to any redress for the hundreds of post office operators who were wrongfully prosecuted on the basis of its faulty IT system. After that admission, Badenoch wrote to the Fujitsu group chief executive, Takahito Tokita, demanding urgent discussions on the proportion of compensation the firm would pay, according to a report by Sky News, which has seen part of the letter. “As you may know, my department is at the forefront of our government’s efforts to right the wrongs of the past,” she wrote. “I am committed to ensuring that postmasters affected get the justice they deserve. This is why the UK government announced new legislation last week, to overturn wrongful convictions and a plan to ensure swifter access to compensation.” She added: “I understand that we are awaiting the conclusions of the [judge-led Sir Wyn] Williams inquiry, but ahead of that I would welcome a discussion with you on the type of response Fujitsu might make and the role you foresee Fujitsu playing towards securing justice for those affected.” The move is likely to put Fujitsu stock under further pressure when Tokyo’s stock market reopens. The share slump began shortly after parliamentary hearings at which Fujitsu issued a formal apology for its part in the affair, and the business minister Kevin Hollinrake predicted that the compensation fund for victims could rise above £1bn. The company is also facing calls to be excluded from lucrative British government contracts. In his testimony to MPs on the business select committee on Tuesday, Fujitsu’s European chief, Paul Patterson, also said the company had known that Horizon suffered from bugs since the 1990s. Despite this, he admitted that the firm had continued to support the Post Office in prosecuting hundreds of post office operators, in some cases leading to innocent people being jailed, or dying before they were able to clear their names. “I think there is a moral obligation for the company to contribute,” Patterson said, adding that Fujitsu was “truly sorry”. However, he said the exact sum could only be determined after the conclusion to the judge-led inquiry into the scandal. He admitted that Fujitsu knew the Horizon system was flawed at the time that the firm was supporting the Post Office in prosecuting more than 900 post office operators. “Fujitsu would like to apologise for our part in this appalling miscarriage of justice. We were involved from the very start. We did have bugs and errors from the start and we did help the Post Office with prosecutions of subpostmasters.” Just hours after that formal apology, Fujitsu’s shares plunged by more than 4%, sending its stock market value down $1bn (£790m) to $24.8bn. Its stock has already fallen by nearly 9% since the start of the year, equating to a $2.35bn loss, but this week’s slump is the largest since the Japanese company began to come under the spotlight in recent weeks. Fujitsu built the Horizon IT system, which produced faulty data that led to prosecutions of more than 900 post office operators for crimes such as theft and false accounting. The Labour MP Kevan Jones told BBC Radio 4’s Today programme on Wednesday: “They knew about them [bugs in the system] but they also supplied the evidence which then the Post Office took prosecutions. What it will mean, is that Fujitsu for the first time have recognised that they will have to pay the bill for some of the compensation and that it will not all fall on the taxpayer.” Fujitsu is also battling the impact of a strike by about 300 of its staff who provide IT support to HM Revenue and Customs. HMRC has said that it has measures in place to avoid any disruption.