Workers at the Port of Felixstowe have today served two weeks' notice before further strike action.
Strikes will begin on September 27 and will last for eight days.
The walkout will clash with action taking place at the Port of Liverpool, which will run from Monday 19 September to Monday 3 October.
Last month, Unite members voted in favour of industrial action by a majority of 92% as part of a pay dispute, with the union holding out for a 10% pay increase.
Following eight days of strikes that began on August 21 - the first since 1989 - bosses at the Port of Felixstowe decided to implement their pay offer last week without any agreement with union leaders.
It is understood Unite put the offer of 7% plus £500 to their members over the last few days and an overwhelming majority of 82% voted to refuse it.
As a result, members have served notice of a second wave of strikes at the Port.
The crossover with action at the Port of Liverpool is considered to be a major cause of concern, with a number of cargo ships potentially being redirected to other ports or back to Europe.
An anonymous Suffolk shipping expert said: “At a time when all of us are facing further financial pressure, the wider impact on associated industries and business in general is unnecessarily damaging.”
Adam Searle, a Suffolk haulage boss, said the last round of strikes cost him £70,000 in just one week.
Following the latest announcement, the Port of Felixstowe have written on their website: "The company has received notice from Unite the union of further strike action from 07:00 on 27 September to 06:59 on 5 October.
"We are very disappointed that Unite has announced this further strike action at this time. The collective bargaining process has been exhausted and there is no prospect of agreement being reached with the union.
"The port is in the process of implementing the 2022 pay award of 7% plus £500 which is backdated to 1 January 2022."
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