But, Alex Walsh, the DMA's head of postal affairs, told printweek.com: "That issue only arises if there is further industrial action," adding that levels of direct mail have so far not suffered as a result of the strikes.

"What the strikes did do was bring the issue up to boardroom level and many [marketers] have brought forward some plans to test other forms of marketing [aside from direct mail]," he said.

The Royal Mail recently published its findings into the impact of the strikes. It found that industrial action resulted in only 78.4% of First Class letters reaching their destination the working day after they were posted, during the three months between early September and the beginning of December. This compared with a 93% success rate in the first quarter of 2007.

Last year's rolling strike action took place after union members rejected Royal Mail's proposals on pay, pensions and modernisation.

Earlier this month, the Communications Workers Union announced it was holding a consultative ballot with its members over the Royal Mail's plans to restructure its pensions scheme. A spokesman for the Royal Mail told printweek.com that the ballot's findings are due to be revealed towards the end of this month. It will be at this point that the union will decide whether or not to pursue further industrial action.