The latest spate of strike action has resulted in walkouts in locations including London, Birmingham, Bristol and Stoke-on-Trent, while a national strike ballot will be conducted in September.

In addition Royal Mail drivers have taken action leading to severely-disrupted services.

Gillian Lyall, communications solutions director at NB Group, said: "For some companies, if they don’t get their mailing out then this can have a severe impact on a client's cashflow if, for instance, they're waiting for orders."

Lyall, who also chairs the DMA's mailing house council, warned: "As a consequence, this could have an effect on the channel clients choose for future campaigns, which could mean moving away from direct mail."

However, she sympathised with the situation claiming that their needs to be a single strategy but said "when you're dealing with a large beast like Royal Mail, there is often no single solution".

According to David Ing, production director at Leeds-based Lick Group, although the strikes haven't affected mailings or had a detrimental effect on client relationships, there is the potential to impact on deliveries.

He said: "Although the strikes aren’t affecting mailings leaving the warehouse, there’s still potential for them to impact upon the client mail landing.

"If the mail is diverted to an affected sorting office, for example, it becomes an issue completely out of our hands."

Dave Ward, deputy general secretary at the Communication Workers Union said the strikes were a response by postal workers to "an incompetent management running their business into the ground".

He said: "Royal Mail agreed in 2007 to work with the union on agreeing modernisation.

"Despite explicit commitments to negotiate they are reneging on that agreement and imposing panic-driven cuts to jobs and services. This is downsizing, not modernisation."