TfL said the delay was due to “the complexity of introducing entirely new trains onto ageing infrastructure”, with some parts of the line around 120 years old.
Harvey added: “The programme of work that we are undertaking to bring these game‑changing trains to London is hugely complex and is underpinned by our steadfast commitment to safety.
“Despite this change in delivery schedule, our teams have been working intensely to push the programme forward.”
Officials said the wider delivery window took into account the remaining work needed to assess whether any final software or systems changes were required.
Aglaja Schneider, joint CEO of Siemens Mobility UKI, said: “As a Londoner, I know how excited everyone is about the new trains coming into service and, whilst there will be a delay, it won’t be long before these trains transform travel for millions of people.”