LU managing director Tim O' Toole said: 'The Waterloo & City line suffers from constant failures and urgently needs upgrading.
LU said it had consulted with businesses and organisations that would be affected by the closure. They agreed with LU that the line should close for five months so that the improvements could be made all at once rather than through a succession of weekend closures over a longer period of time.
London Transport Users' Committee chairman Brian Cooke said: 'The Tube has suffered from decades of under-investment. If the Underground is to improve, then on occasions there will have to be short-term pain to deliver long-term gain.
'Any closure will result in some degree of disruption to passengers but we are supportive of long-term closures as long as they are well planned, deliver the required improvements within the timescales outlined and viable transport alternatives are in place for passengers.'
During the five-month closure, passengers travelling from Waterloo to Bank will be advised to take the Bakerloo or Northern lines to Embankment station and then the District or Circle line to Monument