A prominent opponent of Vladimir Putin has had his hopes of running as a candidate in the 2018 presidential election thrown further into doubt after a court upheld his conviction for fraud.

The Kirov Regional Court rejected an appeal against a five-year suspended prison sentence for Kremlin critic Alexei Navalny, who hopes to run for the presidency next year.

Mr Navalny was found guilty of embezzlement at a trial in February – charges which would bar him from challenging Mr Putin, were he to stand again, in the 2018 election.

He has vowed to appeal the ruling at the European Court of Human Rights (ECHR).

The charges relate to the alleged embezzlement of 16 million roubles (£215,000) from the state-run timber company KirovLes.

Mr Navalny received a five-year suspended sentence and a 500,000 rouble fine (£6,700). A second defendant, Petr Ofitserov, was fined the same amount and handed a four-year suspended sentence.

February’s trial came after the ECHR ruled an earlier trial to be unfair.

Navalny’s campaign manager, Leonid Volkov, insisted at the time that the campaign would go on even though the guilty verdict formally bars him from running.

In a post on Facebook, Mr Volkov said that the Kremlin would ultimately decide whether Mr Navalny will be confirmed as a presidential candidate.

“This is the political decision we need to win by campaigning,” he said.

The 40-year-old former lawyer revealed on Tuesday that he had lost 80 per cent of the sight in his right eye after an attacker threw green liquid in his face last week.

Yulia, wife of Mr Navalny, treats him after the incident (AP)

He was unable to get treatment outside of Russia because of a travel ban.

His supporters have identified the attacker as a member of a radical ultra-nationalist group. Mr Navalny has said the attack was linked to the Kremlin, which has denied any involvement.

Footage of the incident, the second time Mr Navalny had suffered such an assault in as many months, was shown on a pro-government TV channel.

"For now the loss of sight is not irreversible," he said. "I'm being actively treated and there's hope I will be healed."

Authorities have so far not opened an investigation into the attack.

Mr Navalny was jailed in March after he organised massive anti-government protests across Russia.

Thousands of anti-corruption protesters took to the streets of Moscow and other cities demanding the resignation of Prime Minister Dmitry Medvedev.

Police officers detaining Kremlin critic Alexei Navalny during an unauthorised anti-corruption rally in central Moscow (AFP)

Mr Navalny was sentenced to 15 days in prison after a court ruled the political activist had disobeyed instructions from a police officer during the demonstration.

Kremlin spokesman Dmitry Peskov suggested some of those taking part in the demonstrations had been paid. Protesters had been "promised financial rewards in the event of their detention by law enforcement agencies", he said.

Mr Navalny announced last December that he intended to run for the Russian presidency in 2018.

He is arguably Russia’s best known and most popular opposition leader, running on a campaign to clean up Russia’s political system, tackling corruption in politics and the courts, as well as huge disparities in wealth.

However, opinion polls suggest he would stand little chance of beating Mr Putin, who continues to enjoy favourable ratings.