The Waterloo and City line resumed a full weekday service on Monday after 20 months of closure or restricted service.
The underground line, nicknamed ‘The Drain’, takes passengers from Waterloo to Bank.
It had been closed in March last year at the outbreak of the pandemic following government advice to work from home.
In June, TfL reopened the line during weekday peak hours only, with the number of passengers on the line doubling since.
From Monday, the service will return to a pre-pandemic weekday service, with services running between 6am to 12.30am.
Trains will run every five minutes, increasing to three minute intervals during peak times of 6.30am to 9.30am and 4pm to 7pm.
The news was welcomed by the City of London Corporation, which governs the Square Mile.
Its Policy Chair, Catherine McGuinness, said: “The iconic Waterloo & City line has connected the Square Mile with commuters from Surrey and beyond since 1898, so to have it back at full weekday capacity is absolutely fantastic news.”
She added: “TfL’s resumption of a full service on the Waterloo and City line is yet another step in the sustainable recovery of the City, and for London generally.”
The 1.4 mile shuttle line takes just four minutes but, without it, travelling between Waterloo and Bank on the underground requires multiple tubes.
The news comes just days before the Night Tube is set to make a return - but only on two lines.
The Central and Victoria are set to run on Friday and Saturday nights as they did pre-pandemic from Saturday, November 27.
However no timetable has yet been announced for reopening night services on other lines.