The UN Secretary-General, Ban Ki Moon, will visit Burma on Friday in a diplomatically risky effort to win concessions from the country’s military dictatorship.

Mr Ban hopes to persuade the Burmese junta to release political prisoners, including the country’s democracy leader, Aung San Suu Kyi, in advance of an election next year, denounced by opposition groups as fraudulent and meaningless.

But if he comes away without a significant compromise by the regime, he will further undermine the UN’s authority in a country where neither punitive sanctions nor diplomatic engagement have brought any obvious improvements.

The stakes will be all the higher because of the timing of his visit – he will arrive in the former capital, Rangoon, as Ms Suu Kyi goes back on trial for receiving a clandestine visit from an eccentric American.

Mr Ban’s spokesperson, Michele Montas, said yesterday in New York: “The Secretary-General considers that three of the most important issues for the future of Myanmar cannot be left unaddressed at this juncture of the country’s political process.

“These are the release of all political prisoners, including Daw [Mrs] Aung San Suu Kyi; the resumption of dialogue between the Government and Opposition … and the need to create conditions conducive to credible elections.”

Mr Ban’s emissary to Burma, Ibrahim Gambari, was there over the weekend, his eighth visit since 2006, a period in which political and humanitarian conditions have got progressively worse.

Millions were killed or made destitute by Cyclone Nargis last year, the junta violently quashed a peaceful uprising by monks in 2007, and the number of political prisoners has doubled to more than 2000.

Despite international demands that it enter a dialogue with Ms Suu Kyi’s National League for Democracy, the Government has pressed on with its plans for an election in 2010 which will guarantee a continuing role for the army for years to come.

Zoya Phan of the London-based Burma Campaign UK said: “We have had 20 years of UN envoys going back and forth to Burma and nothing to show for it.

“We need Ban Ki Moon to personally take the lead, but he must deliver practical results, such as the release of all political prisoners. Talking to the generals is a means to an end, but so far the UN seems to treat talks alone as a success.”

On Friday Ms Suu Kyi will appear in court once again alongside John Yettaw, the eccentric American well-wisher who swam to her home in May after he dreamed of her assassination. On Monday Burma’s Supreme Court rejected a request by her lawyers to allow two opposition politicians to appear in court to speak in her defence.

Ms Suu Kyi’s detention had been expected to expire last month, and the strange case of Mr Yettaw provides the junta with a pretext for detaining her for longer, until after the elections which it is promising to hold next year. But it may have misjudged the strength of international sympathy for her provoked by the case.

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