Royal Mail workers have started the first of two 48-hour walkouts in a protest over pay and fears of job cuts.
Staff started to walk out from noon
The strike started at noon after last-minute talks between Royal Mail managers and the Communication Workers Union (CWU) had failed to reach a deal.
A second two-day strike by the CWU's 130,000 members is scheduled to begin at 0300 BST on Monday.
Customers have been warned that the strikes mean there will be no deliveries until next Thursday.
It is estimated that the two walkouts will cost industry millions of pounds.
At the centre of the dispute is the CWU's objection to the Royal Mail's 2.5% pay offer and modernisation plans, which it says will put about 40,000 jobs at risk.
Royal Mail says that dramatic reform is needed to survive in a liberalised mail market.
It is worried that the strike will lead to a further loss of business and increase its financial losses.
"We continue to urge the CWU to work with us to modernise the business and to help secure a successful future for the company and its people." the Royal Mail said on Wednesday morning.
Alan Duncan, shadow secretary of state for business enterprise and regulatory reform said: "This is primarily about modernisation," adding that workers needed to agree to accept plans for automation and flexibility.
He warned: "If they don't, they're simply not going to survive."
Businesses have voiced concerned about the impact the strike will have on their customers.
Bruce Henderson, a partner in a motocycle repair firm in Brighton told the BBC that key workers including nurses and police officers relied on the firm for services.
"They are not going to get their bikes back until at least mid to late next week."
He added that using couriers would significantly increase bills for customers.
And Jamie Murray Wells, the managing director of Glasses Direct told the BBC that the strike could cost "thousands and thousands [of pounds] per week" for the firm.
The firm said it would still ensure glasses reached customers but added that the strike could "compromise the whole business".
After Monday's strike, the CWU plans to stage rolling strikes each Monday until the dispute is resolved.
Each CWU member has been asked to walk-out from the start of their shift.
The union's deputy general secretary, Dave Ward, said the strikes were "a proportionate response to an employer that is completely out of control," after five weeks of negotiations.
Royal Mail has decided to start imposing changes on workers' conditions without union agreement from next week.
Postwatch chairwoman Millie Banerjee said: "It is hugely disappointing to watch a great British institution tear itself apart".
The Royal Mail has asked customers to avoid posting mail during the strikes to avoid a backlog, but if they do, to post mail at Post Office branches, which will remain open as usual.
David Frost, director general of the British Chambers of Commerce, told the BBC that the strike could not have come at a worse time for UK firms fearful of a possible global economic slowdown.