Postal workers started a second 48-hour strike Monday as a dispute over pay and restructuring remained unresolved.
Postal staff belonging to the Communication Workers Union walked out at 3 a.m. (0200 GMT) after weekend talks with management failed to break a long-standing deadlock. A similar strike was staged last week.
Royal Mail employees have staged four national strikes and other forms of industrial action since the summer, when they rejected a 2.5 percent pay increase and the Royal Mail's modernization plans, which the union claims would cut 40,000 jobs.
The modernization plans are vital to help secure a successful future for the company, the Royal Mail says.
Business leaders have urged the two sides to keep talking until a deal is reached, and have warned that the dispute is costing the industry millions of pounds.