Post Office staff will take action on 26, 27 and 30 August, in an escalating action over pay, the Communication Workers Union (CWU) announced.
BT and Openreach workers will also hold strikes again. The union said around 40,000 of its members will walk out on 30 and 31 August, following two previous strikes.
Around that number staged a strike at the end of last month and start of this month in the first national telecoms strike since 1987 and the biggest ever among call centre workers.
Union members working for BT and Openreach had voted in favour of industrial action in protest at a £1,500 pay rise.
It will be the fourth round of action by Post Office workers, including those in Crown offices – larger branches – and in administration and the supply chain.
Some of the action will coincide with strikes by Royal Mail employees.
CWU general secretary Dave Ward said: “It’s disgraceful that one of the UK’s wealthiest and most profitable businesses is refusing to pay a fair wage to its employees – the women and men whose hard work and dedication contribute so much to this company’s success.
“At a time when inflation is at its highest level in decades, and when this company is returning ever-increasing profits, paying out enormous amounts to shareholders and to its senior executive, it’s totally unacceptable to treat the workforce in this appalling way.”
Around 115,000 postal workers have previously announced strikes on 26 and 31 August, as well as on 8 and 9 September.
CWU deputy general secretary Andy Kerr said: “Our BT and Openreach members responded magnificently to the first strike call in July and we’re confident they will be every bit as rock-solid in this second bout of action too.
“We remain, as ever, open to negotiations, but in the meantime we are organising and preparing to deploy our pickets all around the UK.”
A BT Group spokesperson said: “We know that our colleagues are dealing with the impacts of high inflation and, although we’re disappointed, we respect their decision to strike.
“We have made the best pay award we could and we are in constant discussions with the CWU to find a way forward from here.
“In the meantime, we will continue to work to minimise any disruption and keep our customers and the country connected.”
The company has said that after “exhaustive discussions” with the CWU it awarded the highest pay rise in more than 20 years.
The increase was £1,500, representing a pay rise of about 5% on average and 8% for the lowest paid, according to BT.
The company has made clear it will not be re-opening this year’s pay review, adding it would work to reduce the impact of industrial action by, for example, postponing any non-essential planned engineering or software updates, similar to what happened at the height of the pandemic and over holidays such as Christmas.