A Russian court has ordered prosecutors to reopen an investigation into the murder of Tsar Nicholas II and his family, despite the fact that the Bolsheviks who are believed to have shot them in 1918 are long dead.

The Russian prosecutor general's main investigative unit this year said it had formally closed a criminal investigation into the killing of Nicholas II because too much time had elapsed since the crime and because those responsible had died.

But Moscow's Basmanny Court ordered the case reopened on Thursday (local time), saying a Supreme Court ruling blaming the state for the killings made the deaths of the actual gunmen irrelevant, a lawyer for the tsar's descendants and local news agencies said.

"This is an important step in our quest for the truth," said German Lukyanov, the lawyer representing Grand Duchess Maria Vladimirovna, a descendant of the Romanov dynasty who styles herself as the heir to the imperial throne.

"The Russian people have the right to know what happened."

Nicholas II, his wife and five children were killed by a revolutionary firing squad on the night of July 16-17, 1918 in the cellar of a merchant's house in Yekaterinburg, a city 1,450 kilometres east of Moscow.

In 2008 they were recognised by Russia's Supreme Court as victims of Bolshevik repression.

The Romanovs say the investigation is needed to resolve a host of questions about the killings.

Remains believed to belong to the tsar and his family were exhumed in 1991 and reburied in 1998 in the imperial crypt of the St Peter and Paul Cathedral in St Petersburg.

The Russian Orthodox Church says it is still unclear whether the remains are in fact those of the last tsar and his family, a view supported by many members of the Romanov family.

The Romanov dynasty ruled Russia for 300 years until the abdication of Nicholas II in 1917, which was followed by the Bolshevik Revolution, civil war and 70 years of communist rule.

A spokeswoman for the prosecutor general's office declined to say whether it would contest the ruling.

- Reuters