The head of London's transport network is to stand down in January - three years earlier than expected.
Bob Kiley is to stand down in January
Former CIA agent Bob Kiley said he had decided to resign as London's transport commissioner after five years as it was "the right time".
Ken Livingstone said he had made an "outstanding contribution in starting to rebuild London's transport system".
Mr Livingstone said Mr Kiley would continue to advise him as a consultant for the rest of his current term.
BBC London's political correspondent Tim Donovan said he believed Mr Kiley's age and the amount of work needed to get London's transport system ready for the Olympic Games were partly to blame.
Mr Kiley will be 77 in 2012 and it was felt a new commissioner should step in to take on the project, he said.
Mr Livingstone also pointed to Mr Kiley's age, telling BBC News the commissioner would still be a key figure behind the scenes but with a smaller day-to-day workload.
It is known that the commissioner has been frustrated by the Public Private Partnership (PPP) contracts.
He believes he has not been able to get the private maintenance firms to carry out improvements on the Tube network as quickly as he would like.
The commissioner said in a statement: "I have decided it is the right time for me to step down as London's Transport Commissioner.
"It has been a privilege to serve as London's first ever Commissioner for Transport.
"Transport for London (TfL) has more than proved it can deliver since it was formed five years ago."
He thanked the mayor for the opportunity to work on the network and TfL employees for their professionalism.
Mr Livingstone said the commissioner's achievements included the introduction of the congestion charge in central London and the creation of two million more bus journeys around the capital each day.