SAN JUAN: Wal-Mart’s Puerto Rico operation is asking the federal court in San Juan to void what it calls an ‘onerous’ and ‘unconstitutional’ tax that unfairly targets the US retail giant. It said in court papers filed on Friday that the increased levy on purchases from US-based Wal-Mart distribution centres by its island entities constitutes one of the world’s highest taxes and pushes its effective tax rate to 91.5 per cent of net income. The tax increase will cost Wal-Mart $155 million over six years and make its Puerto Rico business unsustainable. The suit says that Puerto Rico’s Act 72 specifically targets ‘megastores’ such as Wal-Mart that have US mainland-based related entities because local operations are exempt from the tax.