The party of pro-democracy leader Aung San Suu Kyi will not attend Monday's re-opening of Burma's parliament because of a row over the oath of office for MPs.
Ms Suu Kyi was among 43 MPs elected from her National League for Democracy (NLD) party in by-elections in April.
They want to swear to "respect", rather than "safeguard" the constitution, which they say is undemocratic.
The constitution was drawn up by Burma's former military junta.
"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.
Burma has begun political and economic reforms in the past year, since a civilian-led government ended nearly 50 years of direct military rule.
But this constitution was introduced by the military administration in 2008 and 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 after elections in November 2010 that were boycotted by the NLD because of election laws they said were unfair.
The BBC's Rachel Harvey in Bangkok says that the NLD is taking this stand as a point of principle.
Aung San Suu Kyi has long argued that constitution is undemocratic partly because of the way it preserves a central political role for the armed forces.
But given the dominance of the military-backed party in parliament, change may be difficult to achieve, our correspondent says.
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.
Our correspondent adds that it remains to be seen whether this is a political or bureaucratic oversight, but until that change is made the NLD says it will not take up any seats. For the moment there is no easy way out of the impasse, she says.
The announcement comes just one day after Japan agreed to write off more than $3.7bn (£2.29bn) of debt owed by Burma and to resume development aid. The accord came at talks in Tokyo between the countries' leaders.
Thein Sein's visit to Japan also comes as EU nations prepare to ease sanctions.
According to diplomatic sources, an announcement about the suspension of sanctions - with the exception of the arms embargo - is expected on Monday.
The US and Australia have already eased some sanctions on Burma following the political reforms.