Thames Water infrastructure works on Adelaide Road are to continue until October 24, meaning road closures and diversions will remain in place.
The works have closed the C11 bus route between Belsize Park and Swiss Cottage in both directions, and were expected to be completed by September 26.
Earlier this month, isolated elderly residents criticised the bus diversions that have left them struggling to get to doctors' appointments and collect prescriptions
Thames Water said the extension is due to "delays" and apologised to customers.
A spokesperson said: “We’re carrying out key infrastructure works on Adelaide Road which will strengthen the local water network for future generations. We’re replacing a large water main so we can keep taps running and toilets flushing for our customers.
"This is a major project which is expected to continue until October 24 and we’re sorry for any disruption this is causing for residents.”
Previously, Transport for London's head of bus business development, Tom Cunnington, said the company had tried to "minimise the impact" of the works.
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