Rail and Tube passengers suffered travel disruption today because of problems including another signal failure and attempted theft of lead from a station.
London Underground travellers were hit after a signal failure at North Greenwich crippled services along the Jubilee Line, which is used by more than 650,000 passengers a day.
Caroline Pidgeon, leader of the London Assembly Lib-Dem group and chairwoman of the transport committee, said: "Today's disruption on the Jubilee Line is extremely worrying.
"This line is absolutely critical to ensuring London keeps moving during the Olympic Games, but far from seeing improvements, its reliability has recently been falling.
"The Mayor and Transport for London need to provide details of the actions that will be taken over the next three months to totally turn this around. We need absolute assurances that the Jubilee Line will be a transformed service this summer."
Nigel Holness, LU's Operations Director, said: "The Jubilee Line was temporarily suspended between Canary Wharf and Stratford earlier this morning in order to repair a signal failure at North Greenwich. Services are now operating across the line.
"Staff, including LU's Emergency Response Unit, immediately went to the site to resolve the issue, and our service control staff are now working flat-out to restore normal services this morning. We apologise to customers who have been delayed as a result."
Passengers on Southern services into London Victoria were told that the attempted theft of lead from Arundel station in Sussex was causing delays, while mainline commuters into London's Liverpool Street suffered delays due to overhead power cable problems at Shenfield.