The first part of the two week campaign of staggered strike action throughout Royal Mail is under way.
Postal workers have so far staged two separate 24-hour strikes
Each division within the firm has been allocated a different day to strike, in a move designed to cause continuous disruption throughout the period.
Union members are protesting about pay and job losses that they say will come from the modernisation of the firm.
Royal Mail says the changes are needed for it to stay competitive and vowed to keep delivering mail during the action.
The first action is being taken by staff working in mail centres and cash handling.
Their strike began at 1900 BST on Wednesday, according to the Communication Workers Union (CWU).
Royal Mail called the CWU decision "hugely disappointing", but said it could not change its position. It also said it would have contingency arrangements in place.
"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," a spokesman said.
The Prime Minister, Gordon Brown was asked about the strike in Prime Minister's Questions.
"People have got to accept settlements that make sure inflation is low in the years to come," he said.
The strikes will see each section of the company, from the sorting and collection centres to those who make the deliveries, walking out for two separate 24-hour strikes over two weeks.
The move is designed to cause continuous disruption to the nation's postal service throughout the period at a "minimum cost" to CWU members, union officials say.
The union says the action reflects its growing frustration with the Royal Mail for its failure to take worker demands for job protection and increased pay "seriously".
The CWU fears 40,000 jobs will go as a result of increased mechanisation of the system. It is also objecting to a 2.5% pay offer.
Union officials argue that they support modernisation and the introduction of new technology and automation, but not in the manner that the Royal Mail has outlined.
The CWU says that automation and modernisation can still be achieved without huge job cuts.
Royal Mail insists that change is essential if the business is going to survive against tough competition from a growing number of entrants in the UK mail market.
The group has recently made representations to the UK mail regulator Postcomm to be allowed to lift the price of its bulk business mail deliveries.
It says this will allow it to claw back some of the 40% of market share it has lost to rival operators since it lost its monopoly 18 months ago.
'No pension decision'
Two 24-hour walkouts have been held in the last month - the first national industrial action at the Royal Mail for more than a decade.
Earlier this week Royal Mail denied a report that it plans to cut the pensions of 167,000 of its staff.
Staff would have to work for five more years before collecting their retirement pay, according to a company document published in the Daily Mirror.
Royal Mail said the document was "out of date" and that no decision on pension provision had yet been taken.
But a CWU spokesman said that the letter would "strengthen our resolve".
"The plans are so detailed they would appear to have made up their mind already."