Lift the cistern lid and you should see the water level much higher than normal. You should see the water flowing out the overflow pipe. (or with a modern siphon into the pan) You will need to change the valve. Turn off the water. And flush the toilet.

To fix an overflowing toilet, wait for the water level in the bowl to drop a little or drain out some of the water using a cup. Use a flange plunger to unclog the toilet. If the plunger does not clear the clog, snake the toilet drain line using an auger. An overflowing toilet may fix itself, but that is not always true.

Overflowing cisterns are another issue that homeowners occasionally face. An overflow typically results from a malfunctioning fill valve or float. Either the float fails to signal the valve to stop filling, or the valve itself is defective. In both cases, you’ll need to replace or adjust the problematic component.

One of the most common causes of a running toilet is when overflow water is leaking down into the bowl from the tank through the overflow tube. ‘This tends to happen when too much water is present in the tank,’ says Matthew Jenkins. ‘You can adjust the water level by changing the height of the float.’