Close the toilet’s isolation valve beside or beneath the cistern until the water stops. A round valve normally closes clockwise, while a lever moves to the closed position. Stop if the valve resists gentle hand pressure. Isolation stops the flow temporarily but does not repair the fault.
If the valve is stuck, leaking or missing, do not force it. Use the property’s main water shut-off only if you can identify and access it safely; otherwise, call a plumber. Flush once after isolation to lower the cistern level, and leave the supply off while the toilet is unattended.
Find the likely cause
Carefully remove the cistern lid, if it is designed to lift off, and place it on a padded surface. For a concealed cistern, use only the intended service opening and do not force panels or wall finishes.
Slowly reopen the isolation valve while watching the cistern, ready to close it again:
- Water rises into the overflow: the float may be obstructed or the toilet inlet valve may not be shutting off.
- Water stays below the overflow but leaks into the bowl: the flush valve seal or flush mechanism is probably leaking.
Safe checks
- Float: Check that it moves freely without catching on the cistern wall or other parts. If accessible, lift it gently. If water continues entering while the float is raised, the inlet valve may be faulty.
- Fill adjustment: Adjust only a clearly identified float control and follow the manufacturer’s instructions, as correct fill levels vary by model.
- Button, lever or chain: Check that it returns freely and is not holding the flush valve open. Do not shorten, disconnect or force components.
- Flush valve seal: With the water isolated and the cistern emptied, inspect an accessible seal for debris, misalignment, wear or distortion. Remove loose debris or reseat it only if this requires no dismantling or disconnection.
Do not use excessive force or fit an unverified replacement part. Inlet valves and flush valve seals vary between toilet models.
When to call a plumber
Arrange a licensed plumber if the isolation valve will not close, the inlet valve continues running, a seal or valve needs replacement, or the cistern is concealed, cracked or damaged.
If water is escaping onto the floor or into a wall, isolate the supply immediately. Do not disconnect the inlet pipe, replace fixed plumbing components or dismantle a concealed cistern yourself. Contact our leaking toilet repair team to diagnose and repair the fault safely.