The strike is the legacy of issues left over from 2007's post dispute
Post sent to Northern Ireland may face disruption because of industrial action by staff in Great Britain.
More than 25,000 Royal Mail lorry drivers are beginning a series of walkouts in the coming days in a row over pay and jobs.
The dispute has not yet spread to staff in Northern Ireland.
But NI Communications Workers Union spokesman Lawrence Huston said the "policy of introducing change without agreement" was damaging staff morale.
A spokesperson for Royal Mail in Northern Ireland said: "90% of staff across the UK and all of our employees in Northern Ireland will be working normally and with the exception of local strikes in London on Friday and next Wednesday, the vast majority of customers services will be operating normally over the next few days."
"We will be working hard to ensure that any impact on mail posted to and from the affected areas in GB is kept to a minimum."
The CWU said up to 25,000 of its members in areas including the West Country, London, East Anglia, the Midlands and Scotland will take industrial action, with more disruption threatened in the coming months.
The union's executive will hold a national ballot of its 160,000 postal members in September which could lead to nationwide strikes in October.
A Royal Mail spokesman said the union was failing to acknowledge "clear evidence" of declining mail volumes.
He said: "The CWU's real agenda is to block change and modernisation at Royal Mail and to absolutely oppose on the ground our goal of making Royal Mail a strong and innovative leader in the UK and international postal markets.
"We condemn the CWU for striking over much-needed modernisation and change which has already been successfully implemented by our people in the majority of offices around the UK and is working well."