More postal workers have walked out in wildcat protests in London, Glasgow and Liverpool in an unofficial action at a change in working hours.
The CWU and the Royal Mail cannot reach agreement on modernisation
The walkouts began as an official strike over a long-running row over pay, pensions and jobs ended.
But what are the postal strikes really about, and what do the Royal Mail and union leaders want?
The key issues of contention between the Royal Mail and the Communication Workers Union (CWU) - which represents postal workers - are pay, pensions and modernisation.
Meanwhile, the series of unofficial wildcat strikes focused on working hours.
PAY AND PENSIONS
The CWU says it has been offered a pay increase of 6.9% but that the offer is subject to "unacceptable strings including a reduction in pension benefits".
Planned 24-hour strikes
15 October from 1800BST: Mail sorting offices and airports
16 October from 0300 BST: Deliveries and collection hubs
17 October from 1200 BST: Royal Mail drivers
18 October from 1200 BST: Manual data entry centres
18 October from 1400 BST: Heathrow world distribution centre
The Royal Mail has proposed scrapping its final salary pension scheme and raising the retirement age from 60 to 65.
But the CWU says a 40-year-old worker with 20 years' service would lose more than £60,000 pension benefits if the proposals go ahead.
However, both sides say they are close to achieving a deal on pay and pensions which is acceptable to all parties.
MODERNISATION AND JOBS
Issues surrounding what the Royal Mail terms "flexibility" are the major sticking point in negotiations.
It says it has been losing business to its rivals since the postal market was opened to competition, and urgently needs to modernise its business practices.
Since the liberalisation of the UK postal service in 2005, there are now 17 other companies competing against Royal Mail, especially in the more profitable business mail sector.
The Royal Mail says it has already lost 40% of this corporate market to rivals.
Royal Mail accuses the union of wanting to preserve working practices such as:
Freedom for workers to go home before the end of their shift if they have completed their work
Automatic overtime pay if mail volumes reach a certain level regardless of how many working hours remain in the day
No overlap between functions in the same workplace
Collection drivers can expect overtime pay for doing collections outside their route even if it is done within normal working hours
Overtime pay to cover colleague absence or to help in the sorting office even if within normal working hours
Source: Royal Mail
Adam Crozier, the Royal Mail's chief executive, claims there are still numerous so-called "Spanish practices" used by its workers which are holding back the business.
The term, seen by many as pejorative, has been used in Britain since the Elizabethan era.
Many say it actually referred to Roman Catholic practices, considered of lesser value in Britain at that time.
Such practices include the freedom for workers to go home before their shift has ended if they have completed their designated workload, rather than help out where needed for the remainder of their working day.
Mr Crozier has called on workers to be more flexible and says the Royal Mail is "simply asking people to work the 37 hours and 20 minutes they are getting paid to work".
He has not specified which of the practices he is seeking to change. However, Allan Leighton, the chairman, has guaranteed that some early finishes will remain.
In response, the CWU says postal workers are already being flexible in the way they work.
Union leaders say workers carry more mail than they should, do not take breaks when they have heavy workloads and use their own cars to deliver mail.
The union also claims the modernisation plans, which include later start times, later deliveries and the abolition of Sunday collections, will lead to a loss of 40,000 jobs and take the service backwards.
The wildcat strikes in London, Glasgow and Liverpool are about an unofficial row over shift times.
The CWU says that Wednesday morning's unofficial action was caused by "management's imposition of unagreed changes, particularly over later starts, and reflects the frustration felt by postal workers at Royal Mail's executive action".
New Royal Mail rules, which postal workers say they have not consented to, prevent them starting work before 0600 BST and leaving before 1415 BST.
Historically, most postal workers began their shift between 0500 BST and 0530 BST and were free to go when they had finished their round.
However, the Royal Mail says workers were given warning of changes to start times which it says are necessary if it is to operate legally.
New laws from 1 January 2008 will restrict the speed of many of its vehicles, meaning mail will arrive later at delivery offices and therefore affect start times.