The Grand Alliance decided to drop the port from its EU4 loop with effect from 4 August, while a quayside upgrade at Southampton Container Terminal takes place.
The alliance assured shippers and forwarders that the change to the service would not compromise transit times as cargo could switch to other loops.
But one forwarder said it was expecting delays of up to a week, as consignments would now be offloaded at Hamburg.
"When the vessel discharges in Hamburg, we've then got to wait for a feeder service to the UK which is going to take time. Once it starts to bite, we'll get all sorts of complaints from importers and we're the ones that get it in the neck," he said.
The Southampton upgrade involves the installation of 35m rails in advance of the delivery of two new post-panamax quayside cranes from Liebherr. Work began in April and the port has recently been operating three out of its four deep-sea berths.
The upgrade was scheduled to be completed before the start of the peak season in order to keep disruption to a minimum. But high demand out of China meant the terminal has been working at full stretch all year.
After the new cranes are commissioned in the first quarter of 2008, Southampton will have 13 quayside gantry cranes, of which seven will be able to handle vessels of more than 18 containers across the deck.
Grand Alliances EU4 loop will now have a rotation of Xingang, Qingdao, Pusan, Shanghai, Ningbo, Singapore, Port Kelang, Jeddah, Rotterdam and Hamburg.