What must you do when the amber light is flashing at a pelican crossing?

A pelican crossing is a type of pedestrian crossing. There are also puffin crossings and toucan crossings.

Rule 22 of the UK Highway Code states:

Pelican crossings. These are signal-controlled crossings operated by pedestrians. Push the control button to activate the traffic signals. When the red figure shows, do not cross. When a steady green figure shows, check the traffic has stopped then cross with care. When the green figure begins to flash you should not start to cross. If you have already started you should have time to finish crossing safely.