DUBLIN - A High Court judge ruled Wednesday that Ireland's government should stop delaying a byelection for a parliamentary seat left empty for 17 months, a judgment that further weakened the ability of Prime Minister Brian Cowen to stay in power.

Cowen's unpopular government has blocked byelections in three Irish constituencies because it expects to lose all three. As it is, Cowen barely retains a parliamentary majority needed to pass emergency deficit-slashing measures in December.

The government argued in court that the Irish constitution provided judges no powers to compel governments to fill long-empty seats in parliament.

But Justice Nicholas Kearns rejected the government's arguments, finding instead that Cowen was violating the constitutional rights of voters and political candidates by postponing the byelection in Donegal, northwest Ireland, until next year.

Kearns, the senior judge on Ireland's second-highest court, said the failure to hold a prompt byelection, the typical practice in Ireland, "offends the terms and spirit of the constitution and its framework for democracy."

One of three lawmakers' posts for Donegal South West has been empty since June 2009 when the incumbent, a member of Cowen's long-ruling Fianna Fail party, left to take a seat in the European Parliament.

Cowen currently can depend on just 82 lawmakers in a Dublin parliament run down to 161 members. He is at risk of losing a key vote in December to pass Ireland's next emergency deficit-cutting budget. Failure to pass the 2011 budget at that time would trigger an early national election.

On Tuesday, Cowen's Fianna Fail suffered another parliamentary casualty with the surprise resignation of lawmaker Jim McDaid. He published a letter to Cowen accusing him of preferring to cling to power than to make tough decisions, and appealed for a national election now.

Wednesday's winning litigant, up-and-coming Donegal politician Sen. Pearse Doherty, celebrated his victory alongside colleagues from the Irish nationalist Sinn Fein party on the steps of the Four Courts building in Dublin. He forecast an early demise for Cowen's coalition.

Doherty, who is favoured to win the Donegal South West seat, said Fianna Fail leaders had conspired "to suspend democracy simply because they're doing badly in the polls."

In Dail Eireann, the Irish parliament, Cowen told lawmakers it would be "premature" to offer any detailed response to the judgment until government lawyers could study it.

The government could appeal it to the nation's ultimate arbiter of constitutional law, Ireland's Supreme Court. That would likely delay the byelection into the first half of 2011, the government's aim all along.

Analysts unanimously agree that Cowen must lose his parliamentary majority whenever the outstanding byelections for Donegal South West, Dublin South and Waterford are held. The latter two constituencies have had vacancies since February and March, respectively.

Irish parties in power rarely win byelections even in good times. But Cowen's 4-year-old government over the past year of recession, double-digit unemployment and debt crisis has plummeted to the lowest approval ratings in the five-decade history of opinion polling in Ireland.

Wednesday's judgment did not prescribe a specific time limit for parliamentary seats to lie empty, but noted that the government's own Law Reform Commission has recommended a limit of 90 days.

Doherty said Sinn Fein would introduce a motion Thursday in parliament demanding an immediate byelection in Donegal South West. He said if the government, barring any appeal, would have no choice but to support it. This would mean a byelection sometime between Nov. 22 and Nov. 29, he said.

Cowen's government has insisted it can't focus on byelections at a time when it is trying to craft a four-year plan for Ireland's financial salvation.

Ireland this year is expected to record the worst deficit in modern European history, 32 per cent of GDP, and has been ordered by European Union leaders to slash the deficit back to the euro-zone limit of 3 per cent by 2014.

Uncertainty over Ireland's capacity to keep paying its bills amid recession and a C45 billion ($62 billion) bank-bailout program has fueled a surge in the costs of Irish borrowing on international bond markets. The interest rates on Irish 10-year bonds continued to hover Wednesday near 7.4 per cent, a euro-era high reached the day before.

The government plans to publish a four-year austerity plan later this month that will convince the markets that Ireland isn't on the road to default. The government then hopes to win parliamentary approval for a 2011 budget that could seek more than C4.5 billion ($6.5 billion) in spending cuts and tax increases in this country of 4.5 million.