Prime Minister David Cameron flew to South Africa on Monday for a trade visit, as the phone hacking scandal at home exploded with the resignation of the country\'s top policeman. Cameron was told about the resignation of Paul Stephenson, the Commissioner of the Metropolitan Police, just over an hour after taking off from London on Sunday for a two-day trip to South Africa and Nigeria, aides said. Stephenson quit amid pressure over his ties to the scandal-hit News of the World, but he issued a parting shot at the prime minister over his decision to employ a former editor of the tabloid, Andy Coulson, as his media chief. Downing Street aides admitted they had been caught off guard by Stephenson\'s resignation, but insisted it was right for Cameron to continue with his trip, which involves a high-level business delegation. He has already shortened the visit, his first to sub-Saharan Africa since taking office in May 2010, because of the phone hacking crisis. It was due to last five days and take in the new state of South Sudan as well as Rwanda. \"A big part of (his job) is boosting the British economy and making trade links, and that\'s the purpose of this trip. The prime minister needs to continue doing that,\" a spokesman told reporters travelling with Cameron. Ahead of his arrival in South Africa, where he will hold talks with President Jacob Zuma as well as Desmond Tutu, Cameron called for Africa to boost its intra-continental trade to reduce its reliance on aid. He admitted the issue of an African free trade area, negotiations on which were launched by 26 states last month, was not as headline-grabbing as aid concerts such as Live Aid, but argued it could be far more powerful. \"In the past, there were marches in the West to drop the debt. There were concerts to increase aid. And it was right that the world responded,\" Cameron wrote in an article in South Africa\'s Business Day ahead of his arrival. \"But they have never once had a march or a concert to call for what will in the long term save far more lives and do far more good -- an African free trade area.\" Cameron argued that a free trade area could increase gross domestic product across the continent by an estimated $62 billion a year -- $20 million more than the world gives sub-Saharan Africa in aid. African leaders agreed in South Africa last month to launch negotiations on creating a free trade zone that would include 26 countries with a combined economy estimated at $875 billion (597 billion euros). Cameron is accompanied on his trip by a delegation of 25 business leaders, from the chief executive of Barclays bank, Bob Diamond, to the director of communications and public policy of the English Premier League, Bill Bush. However, commentators at home are questioning the timing of his trip. The phone hacking scandal led to the arrest Sunday of Rebekah Brooks, a former editor of the News of the World, and the shock resignation of Stephenson. In a statement issued by his office late Sunday, Cameron described Stephenson\'s departure as \"a very sad occasion for him,\" adding: \"I wish him well for the future.\" Brooks resigned as chief executive of Rupert Murdoch\'s News International newspaper division on Friday, although she denies wrongdoing. Cameron is friends with Brooks, and has also come under fire for his decision to employ her successor as editor, Andy Coulson, as his media chief until January. Coulson was also arrested earlier this month over hacking. Stephenson quit after pressure over his links with a former deputy editor of the News of the World, Neil Wallis, who was arrested last week, but he took a sideswipe at Cameron over his ties to Coulson. \"Unlike Mr Coulson, Mr Wallis had not resigned from News of the World or, to the best of my knowledge, been in any way associated with the original phone hacking investigation,\" the police chief said.