And while people will receive some mail today, it has been estimated that it could take up to a week for the backlog to be cleared.

Around 4000 local workers joined the 130,000 other front-line staff yesterday in the first UK-wide strike in a decade.

Despite the industrial action Royal Mail have said that all 540 Post Office branches were open and serving customers and that support at its letters operation was "patchy".

However the main sorting office at Mallusk and nine of the Crown Post Offices were affected by the strike.

Earlier this week the Communication Workers Union defended its decision to stage the industrial action saying it was trying to protect workers and the quality of the postal service.

It also accused Royal Mail of refusing to go back to the negotiating table after members voted to reject Royal Mail's 2.5% pay increase saying it was " just above inflation".

A Royal Mail spokesman apologised for the disruption saying it was now focusing on getting services back to normal as quickly as possible.

The spokesperson also said the organisation remained "very willing" to meet the union to explain its modernisation plans but its offer would still stand.

"The postal market is also declining - by 2.5% a year - in the face of competition from electronic communications such as email and broadband development.

"The simple fact is that rivals pay their people 25% less than Royal Mail and they are 40% more efficient than Royal Mail because they have already modernised so their costs and hence their prices are lower and are undercutting Royal Mail's.

"We are very disappointed that the CWU has failed to grasp these realities of the increasingly tough commercial market we face. The challenges won't go away and the longer we put off change, the bigger the challenge Royal Mail and its people face. The union don't seem to realise how damaging the strike is - to customers, to Royal Mail's people and the company."