The Government is being urged to intervene in the postal workers' dispute as a leading business group warns strikes are putting jobs at risk.
Members of the Communication Workers Union (CWU) will launch a fresh wave of industrial action on Wednesday night, which will disrupt mail deliveries for the next two weeks. The union said the aim was to hit Royal Mail hard, at minimal cost to its members.
The Federation of Small Businesses said small firms are being "caught in the crossfire" of the row over pay and jobs.
Spokesman Matthew Knowles said: "Our research has found that 94% of small firms use the Royal Mail exclusively because the other mail providers do not see small businesses as lucrative contracts.
"The economic impact of this strike cannot be under-estimated and it cannot continue throughout the summer without it putting some small businesses and jobs at risk."
The federation urged Business and Enterprise Secretary John Hutton to get the two sides together, warning that livelihoods were now in jeopardy.
Individual postal workers will take part in two 24-hour walkouts, but the action is being staggered over the next fortnight, with different groups of staff taking action on different days.
The union is also stepping up its political campaigning and explaining to MPs why it has rejected a 2.5% pay offer and the Royal Mail's modernisation plans, which the CWU warned would lead to the loss of 40,000 jobs.
The postal organisation said it was "hugely disappointed" that the union was pressing ahead with more strikes.
A spokesman said: "Only part of Royal Mail's workforce will be taking action on any particular day and we will be working through the disruption to deliver letters to our customers as quickly as we can."
