Royal Mail workers are to stage a further 48-hour strike starting at the end of this month in a dispute over pay and conditions.
Some 115,000 members of the Communication Workers Union (CWU) were already preparing to walk out on Thursday and Friday this week.
But they will now also strike for 48 hours on 30 September and 1 October.
CWU general secretary Dave Ward said: "There can be no doubt that postal workers are completely united in their determination to secure the dignified, proper pay rise they deserve.
"We can't keep on living in a country where bosses rake in billions in profit while their employees are forced to use food banks.
"When Royal Mail bosses are raking in £758m in profit and shareholders pocketing in excess of £400m, our members won't accept pleads of poverty from the company.
"Postal workers won't meekly accept their living standards being hammered by greedy business leaders who are completely out of touch with modern Britain.
"They are sick of corporate failure getting rewarded again and again.
"Royal Mail's leadership have lost the dressing room - and unless they make efforts to get real on discussing a pay rise that postal workers deserve, serious disruption will continue."
A Royal Mail spokesperson said: "The CWU's decision to announce further strike action is placing jobs at risk. Royal Mail is losing £1m a day. Strike action has weakened our financial position and is threatening the long-term job security of our postmen and women.
"The CWU has a responsibility to recognise the reality of the situation Royal Mail faces as a business and to engage urgently on the changes required.
"We are now a parcels business. We must adapt old ways of working designed for letters to a world increasingly dominated by parcels and act fast.
"We want to protect well-paid, permanent jobs long-term and retain our place as the industry leader on pay, terms and conditions.
"The CWU rejected our offer worth up to 5.5% for CWU grade colleagues, the biggest increase we have offered for many years.
"In a business that is currently losing £1m a day, we can only fund this offer by agreeing the changes that will pay for it.
"We apologise to our customers for the inconvenience the CWU's continued strike action will cause. We are doing all we can to minimise any delays and keep people, businesses and the country connected."