ZURICH/FRANKFURT/HONG KONG, Jan 17 (Reuters) - After taking over as chairman of Credit Suisse, Antonio Horta-Osorio pledged to develop a culture of personal accountability at the Swiss bank following a string of scandals. That promise ultimately sealed his fate as he was forced to resign after barely eight months in office for flouting coronavirus quarantine rules in both Britain and Switzerland. It marked the end of his attempts to reform Switzerland"s second-biggest bank, still dealing with the fallout from a slew of earlier failings, from spying on executives to investment losses running into billions of dollars. After resigning, Horta-Osorio expressed regret that his "personal actions ... compromised my ability to represent the bank internally and externally". His spokesperson said he would not be speaking to the media. It was just days after promising to clean up the bank by creating a sense of responsibility that Horta-Osorio flew to London in a private jet to attend the Wimbledon tennis finals, breaking Britain"s coronavirus quarantine rules in the process, three people with knowledge of the matter said. When the breach came to light at the end of last year, Horta-Osorio had already been called out for a similar quarantine transgression in Switzerland and the bank was scouring his record for any other violations. An internal investigation also looked at the Portuguese banker"s use of company jets. He had directed a corporate plane to drop him off in the Maldives on the way back from a business trip in Asia, two people familiar with the matter said. The episodes strengthened the hand of senior managers bristling at what they saw as his interference in strategy, making it all but impossible for the chairman to prevail in a tug-of-war with executives at the bank he had been brought in to overhaul. UNFORGIVING BACKDROP Horta-Osorio explained his Wimbledon lapse by saying he was unaware he had failed to get the business waiver for British rules his office had asked for, laying the blame on staff who he said did not inform him, one of the people said. He had intended to take Credit Suisse clients to the tennis tournament and then gave two tickets to his children when customers bowed out due to the pandemic, the person said. But his defence, coming in the wake of a public apology in December for breaking Swiss quarantine rules, fell flat. The breaches took place against an unforgiving backdrop of growing hostility among Credit Suisse"s senior executives, many of whom resented the chairman"s efforts to reform, people familiar with the matter said. Horta-Osorio suspected details of his transgressions were leaked by people in that group, said one of those people, describing "antipathy" towards him within the bank. One person said the chairman had been at "war" with key members of the executive board. Credit Suisse declined to comment on whether there were rifts between the former chairman and the executive board, including Chief Executive Thomas Gottstein. The bank has not published its investigation into Horta-Osorio. Another person who attends the bank"s high-level meetings said Horta-Osorio, unlike his predecessor Urs Rohner, had questioned executives closely when they gathered every quarter to review the bank"s performance. During discussions with management about the second quarter of last year, he persevered with pointed questions about the wealth management business and growth projections, asking to see the data on which the forecasts were made, one source said. "There were quite a few red faces," the person said. He said many senior employees were unhappy with Horta-Osorio"s attempts to make the board of directors more powerful at the expense of executives. "ORCHESTRATED CAMPAIGN" As well as burning bridges with top bankers, the quarantine breaches prompted widespread criticism of the Portuguese banker in Switzerland, where Credit Suisse is regulated and where one in three of 49,000 staff work. One of the people who spoke to Reuters said Horta-Osorio had been taken aback by the hostility he encountered while in Switzerland. Horta-Osorio"s abrupt departure now leaves a question mark over the bank, as its stock price continues to dwindle and amid renewed speculation that it could become a takeover target for its stronger neighbour UBS (UBSG.S), or a foreign bank. David Herro of Harris Associates, one of the bank"s largest shareholders, had backed Horta-Osorio even after the quarantine transgressions came to light, describing his role in turning the bank around as an important reason to invest. "There is an orchestrated campaign because there are constituents who don"t want to see a strong rebounding Credit Suisse," he told Reuters before the resignation, adding that Swiss media had been kinder to his Swiss predecessor. In recent days, Horta-Osorio had tried to salvage the situation, said one person. He had successfully battled through a similar embarrassment as chief executive of Britain"s Lloyds Bank (LLOY.L) after a newspaper reported that he was having an extra-marital affair while travelling on business. In an email sent to Lloyds staff in 2016, Horta-Osorio made clear he had not breached the bank"s rules by mixing business with personal expenses, and showed contrition. "Having the highest professional standards raises the bar against which we are judged," he wrote. "I don"t expect anyone to get everything right all the time. The important point being how we learn from those mistakes." In Switzerland, his apologies rang hollow.