Bitcoin retreated as much as 7 percent to $6,393.70 at 10:16 a.m. in London, extending its 2018 drop to about 55 percent, according to Bloomberg composite pricing. Ripple slumped 10 percent while Ether and Litecoin sank at least 3.9 percent. All but two of the 100 biggest virtual currencies tracked by Coinmarketcap.com recorded declines over the past 24 hours.