The Communication Workers Union (CWU) has issued notice that more than 800 technical workers are to strike, a move that could affect direct mail deliveries.

Dave Ward, deputy general secretary at CWU, said: "This dispute could have a massive impact on the rest of the mail network next week as Royal Mail sites depend on these workers to maintain machinery and buildings."

The strikes, which are due to begin at 6am on 23 September and finish at 6am on 26 September, cover staff employed by Romec that work in technical grades including electronic security and buildings maintenance engineers.

The engineers are threatening to strike over changes being imposed on attendance patterns.

However, a spokesperson for Royal Mail said: "Royal Mail is confident that its letters services will be maintained should any industrial action by Romec go ahead and we are working closely with Romec to ensure back-up services for the entire mail operation are maintained."

Industrial action by Royal Mail is estimated to have cost the direct mail industry around £8m in 2007.

Allan Kerr, group managing director at direct mail printer Fingerprint, said the strikes were likely to affect the work it carries for customers issuing "last-minute" mailings.

He said: "It makes things awkward and can have quite a bad effect on them. Companies will have to try and get their stock to Royal Mail as early as possible."

He added: "Unfortunately, it's becoming something that we have to live with."