The electricity regulator Mr Tom Reeves has taken unprecedented emergency measures to curb the amount of wind power on the electricity grid, following major concerns about the "security and stability of the power system".

Acting on advice from the National Grid, the regulator has halted - with immediate effect - allowing new wind farms to connect to the system. The decision remains in place until year-end and potentially longer.

The advice from the ESB National Grid says the "intermittent" nature of wind power is creating some difficulties. A document was passed this week to Mr Reeves and the Government by the National Grid.

The document says: "Up to quite recently, wind-generated electricity constituted a very small proportion of the total electricity generation in Ireland. Because of this, it was not necessary to impose strict technical performance standards on such plants."

However, the grid says this is no longer the case and concerns have increased.

"While these issues could be ignored with low levels of wind generation on the system, now that definitive plans are in place to significantly and rapidly increase the proportion of wind generation on the system, a number of complex technical issues relating to wind generation must be resolved."

The decision is likely to cause major upset among wind generating companies such as Airtricity, led by chief executive Mr Eddie O'Connor.

Mr Reeves said he was "naturally reluctant to approve emergency type measures of this nature without any prior warning to the industry", but he had to take account of the "critical importance of overall system reliability".

He was responding to a strongly worded request from the ESB National Grid.

Its managing director Mr Kieran O'Brien wrote to Mr Reeves earlier this week seeking the emergency measures.

In his letter, Mr O'Brien expresses serious concern at the recent rapid increase in the amount of wind generation to be connected to the power system.

He says the National Grid has on several occasions warned that no more than 400 to 500 megawatts of wind power should be connected to the power system prior to 2005/06.

However, he says the amount of wind power connected to the system has now reached 700 megawatts (although this figure also includes future connection agreements).

Later in his letter, he states: "This amount of wind generation does, however, pose an increased risk to the security and stability of the power system which the transmission system operator feels exceeds the level normally likely to be accepted by a prudent system operator."

Apart from halting further wind connections, Mr O'Brien requests several other measures:

The CER should review proposed wind policies generally and consult with the industry between now and year end;

Issue no further offers to wind companies until the technical issues have been resolved;

For the wind farms that already have connection agreements, such plants must comply with revised technical specifications;

The CER should consult with the Northern Ireland regulator, Ofreg, to ensure a co-ordinated policy on wind generation for the whole island.