(IDEX Online) - British miner Petra Diamonds reported a $223m loss for the year to June after facing "unprecedented challenges". It follows the $258m loss it posted last year. Revenue slumped 36 per cent to $295.8m (FY 2019: $463.6m). Its Williamson mine, in Tanzania, has been on care and maintenance since April because of coronavirus. The Cullinan mine - source of source of the largest rough gem-quality diamond ever - as well as Finsch and Koffiefontein, all in South Africa, remain open.Petra said that ongoing uncertainties meant production guidance for FY 2021 remained suspended.It had been struggling before the global pandemic and has last month agreed a debt-for-equity restructuring deal with lenders, after failing to attract any buyers. It will will leave existing shareholders with just nine per cent of the company.Chief executive Richard Duffy said: "I am confident that we are now well down the road in repositioning Petra for a successful and sustainable future, building on the operational successes delivered to date and benefiting from a capital restructuring, once complete, that will result in considerably lower levels of debt going forward."