The Liverpool strike from September 19 to October 3, which was announced earlier this month, will overlap a second walkout by workers at Felixstowe, Britain's top container port, which starts on September 27.
More than 560 port operatives and maintenance engineers employed by Mersey Docks and Harbour Company (MDHC) are striking in Liverpool over a pay rise equating to about 8.3%, Unite said.
With inflation having topped 10% in July and projected by the Bank of England to exceed 13% next month, Unite says MDHC's pay offer represents a "real terms pay cut”.
Peel Ports Group, which owns MDHC, said that the union's pay rise demands equate to about 20%.
"We urge the union to work with us at the negotiating table so together we can find a resolution,” Peel Ports said in a statement.
Unite's Liverpool port workers are also striking over MDHC's failure to honour the 2021 pay agreement, the union said in its statement.

The Three Graces, located at Liverpool Pier Head, shot from the Albert Dock. (Photo: Bryan Field, United Kingdom, Shortlist, Open, Architecture, 2022 Sony World Photography Awards)