Postal workers are believed to have voted in favour of strike action over a pay claim by clerical staff.
The 8,000 staff, members of the Communications Workers Union, have been voting on the proposed strike in recent days and the counting of the votes is due to begin tomorrow.
Last night, it appeared that a large majority had voted in favour of the strike.
The clerical staff are seeking a 15% to 20% pay rise in return for increased flexibility and more efficient work practices.