Nearly 95% of all reported trading in bitcoin is artificially created by unregulated exchanges, a new study concludes, raising fresh doubts about the nascent market following a steep decline in prices over the past year.

Fraudulent trading volume has dogged cryptocurrency trading for years, but the extent of the market manipulation has been difficult to determine. Bitwise Asset Management said its analysis of trading activity at 81 exchanges over four days in March indicates that the actual market for bitcoin is far smaller...