Unions at Aer Lingus, the Dublin Airport Authority and Shannon Airport Authority are to begin balloting for industrial action in a dispute over pensions.

The companies share a pension scheme, the Irish Aviation Superannuation Scheme, which has a deficit of almost €800 million.

Three years of negotiations have failed to reach agreement on resolving the deficit, and unions fear that members could see their benefits significantly reduced.

The result of the ballot is not due until 7 February.

Minister for Transport Leo Varadkar has appealed to workers not to go on strike.

He said there would be no public sympathy for a dispute which disrupted St Patrick's Day holiday travel and key rugby matches.

A complex proposal to restructure the scheme was rejected last year by the Pensions Regulator.