De Beers, the world’s biggest diamond miner, is planning for its turnover to halve this year, it emerged on Wednesday, in the latest sign of how the once-mighty group is struggling to cope with a downturn in an industry it no longer controls.

With diamond prices tumbling by at least 30 per cent over the past year, De Beers has cut production at its mines by 40 per cent year-on-year and mothballed its flagship mines in Botswana.