The short version
- Turn off the incoming water at the internal stop tap.
- Switch off the boiler.
- If water is near electrics, keep clear and isolate the power only if you can do so safely.
- Open taps to drain water from the system.
- Contain water with towels or containers without putting yourself at risk.
- Contact a plumber and explain where the leak is and whether the water is isolated.
Find the stop tap without losing time
The internal stop tap is often under the kitchen sink, in a utility space, beneath the stairs or close to where the main water pipe enters the property. Turn it clockwise to close it. Do not force a seized valve hard enough to break it.
If you cannot find or operate the stop tap, tell the plumber immediately. Your water company may also be able to advise about the external boundary valve, but access and responsibility vary.
Water and electricity
Do not touch wet switches, sockets, appliances or exposed wiring. If the consumer unit is in a dry, safely accessible place and you know how to isolate the affected circuit or main supply, do so. Otherwise keep everyone away and seek emergency help.
A sagging ceiling can release a large volume of water or material without warning. Keep out of the area beneath it.
What to tell the plumber
State the property postcode, the room or pipe affected, whether the water is off, how quickly it was escaping and whether electrics or a boiler are involved. Mention access problems and visible damage. You do not need to identify the pipe material or cause unless you already know.
After the leak is stopped
Keep photographs and a note of the timeline if you may contact an insurer. Dry the property safely and watch for water that has travelled into adjoining rooms. Ask the repairer to explain what failed, what was replaced and whether other damaged sections need inspection.
Sources and further information
External guidance can change. Follow the linked organisation where its advice applies to your situation.