Water could continue gushing into West End Lane for another four days - despite the drought declared in London.
The leak at the junction with Iverson Road is sending gallons of water across the surface between West Hampstead's rail stations.
The issue was reported on Saturday morning and despite "road closed" signs, vehicles are continuing to drive through.
Thames Water said it will be fixed between now and August 18.
A drought was declared across London on Friday.
The mayor of London, Sadiq Khan, said: “The National Drought Group has announced that parts of the country have moved into drought today, and I’m urging all Londoners to do all they can to help our city navigate this exceptionally dry spell.
“London’s river and groundwater levels are below average and reservoir levels have fallen significantly. This has been compounded by the prolonged dry weather and the heatwaves we’ve seen in recent weeks.
“While I am urging water companies to act swiftly to stop the leaks which are leading to millions of gallons of water being wasted every day, Londoners can also play their part by saving water as much as possible at home. We need everyone to pull together to help conserve this precious natural resource.
“I will remain in regular contact with Thames Water to ensure Londoners are fully supported during these challenging times.”
Water companies have faced criticism for wastage from leaks in the network.
A statement on Thames Water's website says: "Every day we supply 2.6 billion litres of water, but not all of that gets to our customers. At the moment, we leak almost 24% of the water we supply. We know it’s not acceptable to be losing so much precious water and we’ve got a plan to fix it. It’s not going to be quick, but we’re making progress.
"We work with Ofwat, our regulator, to agree stretching targets for reducing our leakage. Through careful planning, better use of data and new ways of working, we’ve met our target for the last three years.
"This means that we have reduced our base level of leakage by 10.2% (based on a three-year rolling average) since 2019/20. We know we have more to do and reducing leakage continues to be really important so we will continue to focus on delivering against our challenging target of 20.4% by 2024/25. In 2021/22 we fixed over 60,000 leaks, that’s almost 1,200 on average every week."
Thames Water has been contacted for comment.
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