Royal Mail workers have started another 24-hour strike in a continuing row over pay and jobs.
Union members voted overwhelmingly for strike action
It follows similar industrial action on 29 June - which was the first national postal strike in a decade.
What is the dispute about?
Essentially, it is over pay and potential job cuts.
The Communication Workers' Union (CWU) has rejected a 2.5% pay offer and warned that Royal Mail's modernisation plans will lead to 40,000 job losses.
Postal workers' pay should rise to the national average over the next five years, the union argues, but Royal Mail says this would amount to a 27% increase which it could not afford.
What does Royal Mail say?
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.
In order to compete, it has to modernise, Royal Mail argues, saying failure to do so has already cost it government deals and, recently, an £8 million contract with online retailer Amazon.
The company says it is investing £1.2bn in modernisation to ensure it can compete more effectively against its rivlas.
Who will be affected?
Pretty much everybody to some extent, given that large amounts of mail will not be delivered, collected or sorted.
However, it seems that London was worst hit in the 29 June strike.
The Federation of Small Businesses (FSB) has said its members suffer particularly badly from such action because they are more likely to rely exclusively on Royal Mail.
"A cheque delayed in the post can mean the difference between life and death for a small business," an FSB spokesman said at the time of the first 24-hour stoppage.
What are the long-term implications?
Some believe that continued strikes will encourage more businesses to seek alternatives to Royal Mail.
The British Chamber of Commerce says that firms who still rely on cheques may step up their efforts to move to online payments and consider using the services of Royal Mail's rivals.
But if the Royal Mail succeeds in its modernisation plan despite union opposition, it could prove a turning point in the troubled history of post office industrial relations.
How successful was the first strike?
The CWU said there was "overwhelming" support for the strike on 29 June, with 95% of Royal Mail employees joining the industrial action.
However, Royal Mail denied this, saying that about 60% of employees nationally turned up to work as normal, although this fell to about 22% in London. The union dismissed this as "rubbish".
In a separate action earlier this month, counter staff at the so-called "crown" or main post offices held a half-day strike in a row over the transfer of services to WH Smith stores. Royal Mail said the action closed only a handful of post offices.