HSBC chairman Stephen Green is stepping down after being appointed as Trade Minister.He will step down as chairman of the bank before the end of the year, in time to take up his government role in January. HSBC said it has already started looking for a successor and was still on course to have one in place by the end of the year.'The board would like to thank Stephen for his immense contribution and commitment to HSBC over 28 years, in particular his outstanding leadership as chairman during the financial crisis, when HSBC cemented its position as the world's leading financial services brand,' HSBC said.Green as been with HSBC since 1998 and was chief executive from 2003 to 2006.'It is an honour to accept this new role with Her Majesty's Government, serving Britain in the vital areas of trade and investment,' he said.With much of its business focused on the Far East and other emerging markets, HSBC has managed to avoid the worst excesses of the economic crisis and did not turn to the government for a bailout. Green's appointment comes after a difficult search for a Trade Minister, with other prominent business figures turning the post down. He will not be paid for the role.Green has also resigned as chairman of the British Bankers' Association. He is being replaced by Marcus Agius, chairman on Barclays.Names in the frame to replace to Green at HSBC are thought to include chief executive Michael Geoghegan and former Goldman Sachs partner John Thornton.