First, try shutting off the water supply: find the valve on the pipe below the cistern and turn the screw on it from the ‘ 12’ position to ‘3’. If the toilet is built in, it may be quicker to turn off the property’s stopcock (often found under or by the kitchen sink, or in a front or back hall).
HOW TO STOP AN OVERFLOWING TOILET Photo Gallery
Keep a plunger handy in case your overflowing toilet is caused by a blockage in the U-bend. Make sure everyone in the household knows that nappies, pant liners and some wipes aren’t flushable without consequences!
Sometimes it can be the ball valve inside the toilet cistern that is not functioning properly, so the water level may be too high in the tank. Remove the heavy porcelain lid of the tank (if the toilet is built in, remove the section of panelling covering the top of the cistern first). Flush the toilet and watch how the water fills in the tank. Modern tanks have a float connected to the tube that fills the tank; to adjust the flow, pinch the contacts on the side of the float and move them up and down (see image). For older-style mechanisms with a ball valve float that bobs in the water, try gently lifting the arm of the float to cure the problem or adjusting the screw on the lever of the float.
If none of the advice above works, consider calling in a plumber.