Aung San Suu Kyi
Burma's parliament opens Monday amid a boycott by the party of pro-democracy leader, Aung San Suu Kyi because of a row over the oath of office for MPs.
The National League for Democracy (NLD) want to swear to "respect", rather than "safeguard" the constitution, which they say is undemocratic, reports the BBC.
The row comes as European Union diplomats meet to decide whether to suspend sanctions against Burma.
The April 1 by-elections saw Ms Suu Kyi and 42 NLD members elected as MPs.
The constitution was drawn up by Burma's former military junta. It reserves 25% of all seats in parliament for the military.
"Only after the wording in the oath has been changed will we be able to attend the parliament," Ohn Kyaing, NLD spokesperson and newly-elected MP, told BBC Burmese.
The NLD says it is confident the dispute over the parliamentary oath can be settled, says the BBC's Rachel Harvey in Bangkok. But no one appears to be quite sure how or when.
The dispute has come to a head with foreign ministers of EU nations due to meet in Luxemburg.
According to diplomatic sources, an announcement on the suspension of sanctions - with the exception of the arms embargo - is expected.
The US and Australia have already eased some sanctions on Burma.
Burma has embarked on political and economic reform since the transition to a civilian-led government after polls in November 2010 ended nearly 50 years of direct military rule.
The constitution was introduced by the military administration in 2008 as part of that transition. It allocates 25% of seats in both houses of parliament and the state assemblies to the army.
The army and its proxy Union Solidarity and Development Party (USDP) still hold about 80% of seats in parliament, despite the NLD's recent landslide win in the 1 April by-election. The NLD boycotted the November 2010 polls because of election laws it said was unfair.
Aung San Suu Kyi wants, ultimately, to change the constitution because it enshrines the role of the armed forces in politics, says our correspondent.
But changing the oath may require parliamentary approval which would in turn mean securing the support of the military whose power the NLD wishes to curtail.
A similar change in the wording had to be made to the political registration law in order for the NLD to be able to take part in the latest by-elections. They were the first elections the NLD competed in since 1990.