Which London Underground line was originally planned under the name Fleet line?

The London Underground line originally planned under the name Fleet line is the Jubilee line.

The proposed route was called the Fleet line after the River Fleet, one of London’s lost rivers. The name was changed to Jubilee line in 1975, in connection with the Silver Jubilee of Elizabeth II, although the first section opened later, in 1979.

The original Jubilee route ran between Stanmore and Charing Cross. Major extensions subsequently carried the line eastward through Docklands and southeast London, producing the modern route between Stanmore and Stratford.

The Jubilee line is shown in silver on the Underground map. It is one of the newer deep-level lines and is especially associated with the Canary Wharf area, where large modern stations were built during the East London extension. The line’s name therefore reflects a royal anniversary rather than the river that inspired its original planning name.