Shares in Wockhardt plummeted as much as 20 percent on Wednesday after brokerage Macquarie downgraded the stock on concerns that an import ban imposed by the United States over quality issues would last longer than expected. In its warning letter dated July 18, the U.S. Food and Drug Administration said it may withhold approvals for any new launches Wockhardt was planning for the United States until the company addressed its concerns about the Waluj plant. Wockhardt has previously said the U.S. ban would cost the company about $100 million in sales a year. The manufacturing plant has also been banned from exporting products to the UK due to similar reasons. The stock was down 6.8 percent at 614.50 rupees by 0605 GMT compared with a 0.26 drop in the benchmark Mumbai market .