MG MG3 Owner's Handbook Manual page 149

STARTING & DRIVING
In the event of a collision or sudden impact, the fuel cut-off
inertia switch automatically isolates the fuel supply to the
engine, unlocks the doors, flashes the hazard lights and
turns the interior lights on.
The fuel cut-off Inertia switch is located on the right hand
of inner wing in the engine compartment.
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The switch must be reset before the engine can be started.
Reset the switch by pressing the top of the switch (arrowed
in illustration).