The man went overboard at around 5.40am on Saturday near the popular seaside town of Margate. A Royal Navy ship, a coastguard helicopter and two RNLI lifeboats are scouring the waters up to eight miles from Margate Harbour. The tanker had been anchored in the harbour overnight before heading out this morning.
Weather conditions in the region are expected to be harsher this weekend as Storm Dennis rolls in.
Sixty-gun frigate HMS Winchester is believed to have responded to a relay call and is in the area, according to ship tracking data.
The urgent search operation comes as communities across the UK brace for the arrival of the storm on Sunday - just one week after Storm Ciara battered the nation.
A distress call was received by the coastguard at 5.41am and rescuers scrambled to the area.
A coastguard spokesman said: "RNLI lifeboats from Ramsgate and Margate have been sent, along with the search and rescue helicopter from Lydd.
"A police RHIB (rigid hull inflatable boat) is also on scene.
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The distress call is believed to have come from B Gas Margrethe, a 99-metre Maltese tanker.
The Met Office has issued a number of warnings as the country braces for the onslaught of the latest storm.
These include five amber warnings for rain which will expire on Sunday night.
Weather experts have warned people to prepare for “very heavy rain, flooding and disruption to travel” on Saturday.
Defence Secretary Ben Wallace said the government will use "all means" available to help flood-hit communities.
A deployment of 75 soldiers from the 4th Battalion, Royal Regiment of Scotland, was sent to support the Environment Agency's efforts in Calderdale, West Yorkshire, which, for a second weekend in a row, was expecting floods.