YANGON (AFP) - Aung San Suu Kyi's opposition party on Wednesday admitted it "cannot win" its battle to scrap parts of a junta-era constitution that bars her from Myanmar's presidency, decrying a parliamentary decision to postpone amendments until after 2015 elections.

Party spokesman Nyan Win said an effective army veto in parliament meant it would not prevail in its efforts to overhaul the charter in an ongoing crunch debate in the legislature, in which representatives of the powerful military have lined up to voice opposition to large-scale change.

"Calculate the ratio mathematically. We cannot win (the fight to change key sections of the constitution)," he told AFP, listing both the clause that bars Suu Kyi and the one that gives the military the last say on amendments.

"So, why are we working for it? Because we believe in democracy," he added, in some of the party's most downbeat remarks on a constitution that many believe was specifically designed to thwart Suu Kyi's political rise.

Legislators will choose a new president after the general election next year. But the veteran democracy campaigner cannot stand for the top post because the constitution bans those with a foreign spouse or children.

Her late husband and two sons are British. Myanmar's parliament speaker Shwe Mann on Tuesday announced a May referendum on any charter amendments voted through by parliament in the current heated debates in the capital Naypyidaw.

But he said it was "impossible" to implement any changes until after crucial November 2015 polls, seen as a test of the country's transition from outright military rule, a process that began in 2011.