The original quartet of Kathleen Dawson, Adam Peaty, James Guy and Freya Anderson were more than two seconds clear of the rest of the field in the heats, setting a new European record, and Team GB, with the addition of Hopkin for Anderson, eased to victory on Saturday morning.
Here are some pictures from an historic golden moment ...
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Anna Hopkin (right) of Team Great Britain celebrates winning the gold medal in the Mixed 4 x 100m Medley Relay Final at Tokyo Aquatics Centre
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Gold medalist Kathleen Dawson of Team Great Britain (right) places the medal of Anna Hopkin of Team Great Britain around her neck during the medal ceremony
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