And now, in what will be a severe embarrassment for minister Seamus Brennan in the run-up to the election, over 400 clerical staff working in the department's Sligo offices have decided to take industrial action from Monday week to publicise the growing problem.

The staff, members of the Civil and Public Service Union, claim the department has tried to keep the problem quiet and has instructed staff to say nothing about the malfunctioning computers to pensioners querying the delays.

"Staff in Sligo are fielding thousands of irate calls from pensioners who have suffered delays of up to six months getting their pension. But we cannot tell the pensioners what the real problem is and this is putting immense strain on staff, " said Derek Mullen, assistant general secretary of the CPSU.

"We want the department to put more resources into fixing the problem with the new system rather than putting more and more staff on the phones to fob off the queries, " he said.

Mullen added that management was totally underestimating the problem, which is growing by the week, and appeared reluctant to address it.

"The department is adopting the same approach as Nero who fiddled while Rome burned, " he said.

A department spokesman confirmed that there had been delays in issuing new pension payments because of teething problems with the implementation of the new computer system . . . the Service Delivery Modernisation (SDM) platform which is designed to streamline pension and other social welfare payments.

But the spokesman put the backlog of pensioners at around 10,000 and said 2,500 of these were people who had applied for a pension before they had reached pensionable age.

He added that management had twice put forward proposals to resolve the problem to the Sligo staff, but these had been rejected by the union.

"The department is continuing to make every effort to reduce the backlog, " said the spokesman. Ongoing payments are not affected.

However, the CPSU said this was an "understatement" and that the backlog was 14,500, not including around 3,000 European pensioners living in Ireland who are entitled to receive pensions from their own country through the Irish welfare system.

Mullen added that when the staff first balloted for action last month over the delays, management threatened to withhold from them a 2% pay increase due from 1 June next under the national pay agreement, Towards 2016.

But management later withdrew this threat for fear that it would exacerbate the situation, and cause both the dispute and the computer problem to become public, Mullen claimed.

Mullen said the union did not want a dispute, particularly in the run-up to a general election, but that it had been forced into this action by the department which was trying to bury the real problem at the expense of its staff.