The cryptocurrency analyst website CoinDesk said the digital currency had fallen by about US$1700 or nearly a third of its value in seven days.
CoinDesk reported a value of US$3745.89 on Saturday which it said was a fall of 81% from the all-time of US$20,000 in December last year.
But other cryptocurrencies like XRP, ethereum and dash were all posting increases in price, CoinDesk said the broader market was still posting losses.
A week earlier, the cryptocurrency market was valued at US$182 billion but this had now fallen to US$124 billion, CoinDesk said.
CNBC said the recent downturn had begun in mid-November when bitcoin first plunged below US$6000. But the currency had experience a relatively stable October in the US$6400 range when the general stock market was volatile.
Contributing to the downward pressure on digital currencies has been a crackdown by the US Securities and Exchange Commission which announced its first penalties against an exchange recently.
Last month, a study by data analyst Juniper Research said the value of annual bitcoin transactions was expected to slide further in the first half of 2019.
The study said daily transactions had fallen from an average of 360,000 in late 2017 to 230,000 in September.
Juniper claimed at the time that bitcoin had been hit by weakness in a number of fiat currencies and also continuing uncertainty around Brexit outcomes and the ongoing trade disputes between the US, China and the EU.