Thousands of postal workers have walked out in a second day of action, joining telecoms workers, who are are also out on strike, as unions push for higher wage rises for their members.
On Wednesday 115,000 Royal Mail workers were on strike while 40,000 BT and Openreach workers stayed out, in action that began on Tuesday.
The postal worker strikes will disrupt delivery of packages and letters.
Royal Mail staff are planning further strikes on 8 and 9 September.
Members of the Communication Workers Union (CWU) at the Royal Mailsaid they werestriking after rejecting a pay rise of 2%, against a backdrop of the rising cost of living.
The Royal Mail said the strikes could damage the firm.
"The CWU's self-centred actions with the wider trade union movement is putting jobs at risk, and making pay rises less affordable... making Royal Mail's future more uncertain than at any time in its long history," a Royal Mail spokesperson said.
The CWU - which says it is the biggest strike in the UK since 2009 - has called on Royal Mail to increase wages to an amount that "covers the current cost of living".
Similarly, the CWU staff at BT Group are striking against pay deals that do not keep pace with the cost of living.
"The reason for the strike is simple: workers will not accept a massive deterioration in their living standards," said CWU general secretary Dave Ward.
Prices in the UK are rising at their fastest for 40 years, with inflation at record levels.
The strikes will affect the delivery of post, including premium paid services like Royal Mail Special Delivery, which guarantees arrival by 1pm the next day.
"Unfortunately, we will not be able to provide any compensation for items that are late as a result of industrial action," a Royal Mail spokesperson told the BBC.
"For Special Delivery Guaranteed items, the guarantee will be suspended for items that are sent the day before the strike, until the industrial action is over. Customers' terms and rights regarding loss and damage of mail remain unaffected," the spokesperson added.
The CWU said during the first day of strike action on 26 Aug that its members had lost confidence in Royal Mail.
"They've lost faith in the leadership and people will understand that when they see the way that the company have conducted themselves," the CWU's Dave Ward said at a strike site in London."We are going to fight hard to get our members the pay deal that they deserve," he added.
Royal Mail's chief executive Simon Thompson told the BBC on Friday: "Our reality is that the Covid bubble has burst, and we can see the economic situation around us all. And our reality today is we are losing £1m a day.
"The change that we need is to pivot our business from a business that was built for letters into a business that can now win in the parcels market."
The BBC has contacted the CWU for comment.
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