Labour's leadership battle lurched to the Left again today as Wes Streeting called for a wealth tax.The former health secretary - once seen as the Blairite candidate - demanded a levy on the 'rich' that he claimed could bring in £12billion a year.But the idea was immediately criticised by economists, who warned that in reality it risks reducing Government revenues and killing investment.Downing Street also played down the prospect, insisting the system already ensures those with 'the broadest shoulders pay their fair share'. The intervention comes as nervous markets brace for a 'bidding war' in Labour that could drive the party towards more extreme policies.Big beasts are gearing up for a contest to replace Keir Starmer within months, whether or not Andy Burnham wins the Makerfield by-election on June 18.Rachel Reeves has already pushed the tax burden towards a record high, squeezing growth. In under two years the Chancellor has imposed measures raising an extra £75billion annually. And there are fears a new spending bonanza could send the costs of servicing the UK debt mountain spiralling. Wes Streeting - seen as the Blairite candidate - called for a levy on the 'rich' that could bring in £12billion a year