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 ...
5.
Left to right: Gold medalists Kathleen Dawson, Adam Peaty, James Guy and Anna Hopkin of Team Great Britain pose after the medal ceremony for the Mixed 4 x 100m Medley Relay Final at Tokyo Aquatics Centre
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Left to right: Gold medalists Kathleen Dawson, Adam Peaty, James Guy and Anna Hopkin of Team Great Britain pose after the medal ceremony for the Mixed 4 x 100m Medley Relay Final at Tokyo Aquatics Centre