According to londonist.com, London's longest tunnel – in fact the longest tunnel in the UK, longer even than the Channel Tunnel – is the Thames Water Ring Main, at 80 kilometres (50 miles).
The question did point us towards a tube line however, and Londonist agrees that the longest tube tunnel is on the Northern Line, at 28 kilometres (17.4 miles). This would appear to be London's second longest tunnel, after the Water Ring Main.
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