By Steven Eke
BBC Russian affairs analyst
With prosecutors announcing a large number of alleged falsifications in signature lists, Mikhail Kasyanov's attempt to secure registration for Russia's presidential election appears to have failed.
Opinion polls suggest Mr Kasyanov would stand no chance in the race
Since his dismissal from the post of prime minister in 2004, Mikhail Kasyanov has emerged as one of the most consistent critics of Vladimir Putin's style of governing.
In near-fluent English, the gravel-voiced Mr Kasyanov accuses President Putin of destroying independent media and political pluralism, and of wrongly believing that the state can control all the vital political and social processes in society.
Mr Kasyanov was born outside Moscow in 1957, and studied to be an engineer and economist. Like many other latter-day Russian converts to market economics, he worked for many years in Gosplan, the USSR's powerful, monolithic state planning committee.
Whether Mr Kasyanov had always disliked Vladimir Putin's rule, or whether his animosity was sparked off by his dismissal, remains open to question
In May 1999, the twilight of the presidency of Boris Yeltsin, he was appointed Russian finance minister.
He thus became one of the last top figures to become popularly associated with "the family", the extended network of Yeltsin relatives and supportive oligarchs now widely blamed for many of Russia's travails during the 1990s.
Rise and fall
Nonetheless, in January 2000, President Putin promoted Mr Kasyanov to the post of first deputy prime minister. His portfolio was widened, encompassing social development, industrial policy, management of finances and reserves.
The split between Mr Kasyanov and Mr Putin was harsh and irrevocable
Within five months, he had been made prime minister.
He set out three key priorities - the fight against corruption; tackling Russia's considerable drugs problem; and protecting the rights of property owners.
In one of the seeds of his eventual downfall, he also called for an end to the political use of the police and security services in what were otherwise purely business disputes.
The campaign against the oil giant Yukos and its former owner, Mikhail Khodorkovsky, was a watershed. There were serious, public disagreements between Mr Kasyanov, on the one hand, and Mr Putin and the security services, on the other.
While Mr Kasyanov said the affair was ruining Russia's image and frightening off potential investors, leading prosecutors loyal to the president accused him of exerting undue pressure on them.
At the same time, Mr Kasyanov took personal credit for the improvement in Russia's economic condition.
By the end of 2003, when growth and inflation figures finally made for happy reading, it sometimes seemed that Mr Kasyanov, a man of considerable personal charm, had emerged as a challenge to Vladimir Putin himself.
And so when Mr Putin sacked his prime minister, in February 2004, the split was harsh and irrevocable.
Whether Mr Kasyanov had always disliked Vladimir Putin's rule, or whether his animosity was sparked off by his dismissal, remains open to question.
Challenging status quo
Other leading opposition figures have at times publicly questioned Mr Kasyanov's probity.
Liberal leader Grigory Yavlinsky once famously suggested that asking Mr Kasyanov to fight corruption was similar to putting a vampire in charge of a blood-bank.
To this day, many ordinary Russians refer to Mr Kasyanov as "Misha-Two-Percent".
However, Mr Kasyanov very quickly emerged as an outspoken critic of Mr Putin and his security service entourage. His message was warmly received in the West, but not at home.
In April 2006, he was elected chairman of the PDU (People's Democratic Union), one of the groups actively trying to build a wide opposition coalition. He helped organise the so-called "dissenters' marches", which were violently suppressed by police.
Opinion polls suggest he is deeply unpopular and would stand no chance whatsoever of being elected president.
Mr Kasyanov himself has stated in interviews that his primary purpose is to reinvigorate political debate, and to stimulate what he describes as the "third of the population that thinks", to challenge the status quo.