Chinese artist and government critic Ai Weiwei says he has decided to challenge the $2.4m (£1.5m) tax bill served on him by the authorities last week.He will have to put up more than half of that amount as collateral to allow him to start legal proceedings.Most of that money will come from donations that flooded in after the artist revealed how much he had been asked to pay.He said the case could take a year to resolve.Ai Weiwei, who helped design the iconic \"Bird\'s Nest\" stadium built for the Beijing Olympics, was sent the bill at the beginning of this month. He was given 15 days to pay.The authorities said earlier this year that the artist had \"evaded taxes\".That accusation came after he had been detained without charge for nearly three months, leading to an outcry both inside and outside China.His mother, Gao Ying, was intending to sell her house to help him.\"The deadline for payment is so tight that not even robbing banks could help,\" she said in an open letter.\"The only thing I can do is use my home. I am willing to first mortgage it and then auction it.\"But the procedures needed to put that plan in motion could not be completed before the artist needed the money, either to pay the bill or use as collateral.\"So we\'ve decided to use the money people loaned us,\" said Mr Ai.\"Once we\'ve put in the collateral, it means we can start to go through the courts.\"The artist needs 8.5m yuan ($1.3m, £840,000) collateral.People started donating money to Mr Ai when he revealed how much the tax authorities wanted him to pay in back taxes and fines. Those donations stood at about 7m yuan.Many believe the tax bill was served because the artist is an outspoken critic of the government - and the Chinese Communist Party that runs it.He uses his international fame to shed light on controversial issues that are rarely aired in public in China.