The United Nations human rights envoy to Burma has cast more doubt on the legitimacy of next month's elections.
In a report, Tomas Ojea Quintana said conditions for genuine elections were limited, with election laws restricting freedom of expression and assembly.
He called on the military government to release more than 2,000 political prisoners, including the detained pro-democracy leader Aung San Suu Kyi.
The military government said the report was based on fabricated information.
The polls will be held on 7 November. Ms Suu Kyi's party, the National League for Democracy (NLD), won the last election in 1990 but was never allowed to take power.
Aung San Suu Kyi has spent most of the years since then under house arrest. The NLD was forced to disband earlier this year after it said it would boycott the elections because of laws favouring the military.
These elections are meant to be part of an orderly transition from military rule to democracy but, the UN envoy said, the process remains deeply flawed.
He said election laws had further tightened long standing restrictions on freedom of expression and assembly, and listed obstacles faced by political parties which were not backed by the government.
Burma's military government rejected the report, saying it prejudged the election from a negative perspective.
Mr Ojea Quintana also said justice and accountability were essential for the transition that Burma was making.
He stood by his proposal for a commission of enquiry into possible war crimes carried out by the military rulers, despite opposition from some UN member states who see such a move as counter-productive.
Burma is not on the Security Council agenda, and a UN diplomat said there are no immediate plans for any visits by UN officials to avoid the risk of appearing to endorse the election process.
Officials at the UN headquarters in New York have said they are waiting to see whether there will be opportunities for constructive engagement after the poll.