Mr Vincent has been under pressure to step down since the beginning of last month, when the company made a surprising climbdown at an industrial tribunal, which resulted in more than 270 dock workers receiving compensation for unfair dismissal.
The tribunal was unexpectedly halted on its second day when the company conceded that it had dismissed the dockers unfairly following a change in working practices before the takeover by Mersey.
The dock workers received pounds 10,000 each in compensation but are planning to take further action in connection with the shares they were forced to sell back to the company on their dismissal.
The workers each sold their shares for pounds 2.50 on the back of a valuation from KPMG Peat Marwick, only to find that Mersey Docks offered pounds 37.25 a share a few months later.
The Independent on Sunday revealed last month that Mr Vincent bought more than 100,000 shares in Medway from former directors only three months before the pounds 103.7m takeover by Mersey Docks, making a profit of about pounds 3.6m on the purchases.
Medway said that Mr Vincent had resigned voluntarily to pursue private interests outside the ports industry.
A company spokesman added that he would be retained as a consultant to the group for the next 12 months to help with issues relating to the change of ownership of Medway.
He received no pay-off from the group but will be paid an undisclosed consultancy fee, which the company described as 'modest'.
Mr Vincent was not available for comment. His present executive responsibilities have been assumed by Bernard Cliff, port operations director.
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