Mr Pinheiro said there were no signs of political change in Burma
Burma's referendum next month will be a "ritual without real content" unless international monitors are allowed in, a top United Nations official has said.
Paulo Sergio Pinheiro, the Special Rapporteur on Burma, also accused military rulers of a clampdown on people campaigning for a "no" vote.
The referendum, set for 10 May, is on whether to adopt a new constitution.
Leaders say it will pave the way for elections by 2010, but critics say it is aimed at entrenching military rule.
The charter was drafted by the generals without input from the pro-democracy opposition.
It allocates a quarter of seats in parliament to the military and bans anyone who has been married to a foreign national from holding office - ruling out detained pro-democracy leader Aung San Suu Kyi.
Her National League for Democracy has called on people to vote against the referendum.
Speaking in Brussels, Mr Pinheiro said that the referendum would not have any credibility if opponents were prevented from speaking out.
Burmese troops used force to end anti-government protests in September
"How can you have a referendum without any of the basic freedoms?" he was quoted by the Associated Press news agency as saying.
"It would be important to have international observers to validate the referendum, because if not it would be just a ritual without real content."
In a separate interview with Reuters news agency, he accused the Burmese government of detaining "no" campaigners, and said that he saw no signs of political change there.
"If you say a real political transition process is taking place in Myanmar (Burma), this would be almost offensive to countries in Asia like the Philippines and Indonesia or Thailand that passed through a transition process to democracy," he said.
Mr Pinheiro last visited Burma in November 2007, weeks after a military crackdown on anti-government protests left at least 31 people dead.
The Burmese government has since then refused to allow him back in.
The full text of the constitution went on sale in government bookshops on 9 April.