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Gas Leak in London? What to Do in the Next 5 Minutes
A gas leak is a life-threatening emergency. Get everyone out first. Read this page from outside, not inside your home.
National Gas Emergency handles the immediate crisis. We handle Gas Safe repairs after the supply is declared safe.
Identification
What Does a Gas Leak Smell Like?
Natural gas is completely odourless in its natural state. Gas suppliers add a chemical called mercaptan -- a sulphur compound -- specifically to make leaks detectable by smell.
The smell: rotten eggs or rotten cabbage
Mercaptan has a very distinctive odour -- most people describe it as rotten eggs or rotten cabbage. Once you have smelled it, you will recognise it immediately. It is added in concentrations specifically chosen to be detectable at very low gas concentrations, well below the explosive threshold.
Smell strength is not proportional to leak severity
This is the most dangerous misconception. A serious, dangerous leak may smell faint if there is ventilation. A minor leak may smell strong in a confined space. Do not judge how serious a leak is by how strongly you can smell it. If you notice the smell at all, treat it as a potential emergency.
Carbon monoxide has no smell
A gas leak that results in incomplete combustion -- burning with insufficient oxygen -- produces carbon monoxide (CO). CO is completely odourless and colourless. It is potentially fatal. A CO detector is essential in any property with gas appliances. If family members or pets are feeling unwell (headaches, dizziness, nausea) without explanation, CO poisoning should be suspected even if there is no gas smell.
Warning Signs
Signs of a Gas Leak Beyond the Smell
Hissing or whistling
A hissing or whistling sound near gas pipes, the meter, or any gas appliance is a strong indicator of escaping gas under pressure.
Dead or discoloured vegetation
A patch of dead or yellowed grass or plants above an underground pipe run can indicate a slow mains leak below ground.
Bubbling in puddles
Bubbling in standing water near your gas meter or along a pipe run means gas is escaping underground and rising through the soil.
Dust blowing from ground
Dust, soil or debris blowing from a hole in the ground near gas infrastructure indicates significant underground leakage.
Yellow or orange pilot flame
A boiler or cooker pilot light that burns yellow or orange (instead of crisp blue) indicates incomplete combustion -- a serious warning sign.
Unusually high gas bills
A sudden or unexplained increase in gas usage that does not match your heating demand can indicate a slow leak losing gas continuously.
People or pets feeling unwell
Unexplained headaches, nausea or dizziness among family members or pets can indicate carbon monoxide produced alongside a gas leak.
Critical Warning
What NOT to Do if You Suspect a Gas Leak
Natural gas is explosive between 5% and 15% concentration in air. Any ignition source -- including an electrical switch -- can trigger an explosion before you can smell the gas at dangerous concentrations.
Emergency Process
What Happens When You Call 0800 111 999
The National Gas Emergency Service is operated by National Grid and Cadent. Here is the process from the moment you call to when gas is restored.
Call from outside
Use your mobile outside the building. Dial 0800 111 999 and give your full address. The line is free and available 24 hours a day, every day of the year.
Engineer dispatched
National Grid or Cadent dispatch a gas emergency engineer. Their priority target is arrival within 1 hour. In practice, response in London is often faster.
Engineer locates source
The engineer uses gas detection equipment to trace the source of the leak -- boiler, pipework, meter, or mains connection.
Supply made safe
The engineer isolates or caps the supply to make the property safe. They may turn off the gas at the meter or cap an individual appliance.
Warning notice issued
You receive a written warning notice describing the nature and location of the fault. This document is what you hand to the Gas Safe engineer who does the repair.
Gas Safe repair required
Cadent makes it safe but does not repair appliances or internal pipework. You must engage a Gas Safe registered engineer to carry out the repair before gas is restored.
Important: Cadent makes it safe. We make it work again.
The National Grid / Cadent emergency engineer will isolate or cap the gas supply. They will not repair your boiler, pipework or appliances. After they leave, you need a Gas Safe registered engineer to carry out the repair before gas can be restored. That is where we come in.
After the Emergency
Gas Safe Repair: Getting Your Supply Restored
Once Cadent or National Grid have made the supply safe, you will have a warning notice describing the fault. The gas will not be restored until a Gas Safe registered engineer carries out and certifies the repair.
Read the warning notice
The notice identifies the specific fault (e.g. 'faulty boiler gas valve', 'corroded pipe joint at meter connection'). Share this with the engineer when you call -- it allows them to arrive with the correct parts and saves time.
Verify Gas Safe registration
Before any engineer touches your gas supply, verify their registration at gassaferegister.co.uk. Enter their Gas Safe ID number -- each registered engineer has a card with a unique ID. This is a legal requirement, not a formality.
Gas Safe certificate after repair
Once the repair is complete, the engineer issues a Gas Safe certificate (formally: a Landlord Gas Safety Record or CP12 for commercial, or a Gas Safety Certificate for domestic). This document confirms the appliance or pipework is safe and compliant. Keep it.
Root Causes
Common Causes of Gas Leaks in London
London properties have specific risk factors: older pipework, hard water affecting fittings, and a high proportion of Victorian housing stock with original gas runs.
Faulty boiler gas valve
The most common appliance fault. The gas valve inside the boiler can fail to seat properly, allowing a small continuous leak. Often presents with a faint smell near the boiler.
Corroded or damaged gas pipe
Older properties -- particularly Victorian and Edwardian London homes -- have copper or iron pipework that can corrode at joints over decades. Pipe runs inside walls are difficult to inspect without specialist equipment.
Cracked flexible hose behind the cooker
The braided flexible hose connecting your cooker to the gas supply has a limited service life. Older hoses crack and split with age, especially where they flex repeatedly as the cooker is moved.
Faulty or poorly fitted gas fittings
Work carried out by unregistered tradespeople is a significant source of gas leaks. Compression fittings not fully tightened, incorrect tape on threaded joints, or the wrong fittings for the pipe type all cause slow leaks.
External gas main damage
Construction work, subsidence or ground movement can damage the mains supply pipe. If you smell gas outdoors near a construction site or along a road, report it to National Grid immediately.
Repair Costs
Cost of Gas Leak Repair in London 2026
The emergency response from National Gas Emergency is free. The repair by a Gas Safe engineer is charged as follows. All prices include VAT and a Gas Safe certificate on completion.
Fixed quote given before any work starts. Call-out fee waived on jobs over 2 hours.
Gas Leak Questions
Frequently Asked Questions
Gas Supply Restored? Book a Gas Safe Repair
Gas Safe registered. London-wide. Rapid response.
Once Cadent or National Grid have declared the supply safe, call us to carry out the Gas Safe repair and get your gas back on. We cover all London boroughs and arrive with the most common parts already on the van.
Gas Safe registered · No call-out fee on jobs over 2 hours · All London boroughs