Mr Kasyanov is one of Vladimir Putin's rare public critics

An official said Mr Kasyanov's Russian People's Democratic Union had not properly completed the documents.

Mr Kasyanov, premier between 2000 and 2004, has become one of the rare public critics of President Vladimir Putin.

His group, the RPDU, condemned the decision not to register it as "unwarranted" and "erroneous".

But a spokeswoman for the registration service, Galina Fokina, said the movement's decision to call itself a union was both misleading and illegal.

"You cannot understand from it whether it is a movement or a union, which violates the law," she said, according to AFP.

She denied the decision was political, pointing out that the RPDU could reapply as many times as they wished.

Mr Kasyanov is the only Russian politician to have declared his intention to run for president when Mr Putin steps down after he completes his second term in office in 2008.

The RPDU said the decision was not unexpected.