Passengers had been promised 20 to 40 minutes being cut from journeys next year following the introduction of new trains.
However, the UK Department for Transport (DfT) said this would not now happen until 2021.
London North Eastern Railway (LNER) will introduce faster-accelerating Azuma trains on part of the line in May, which are due to run as far as Edinburgh by “late summer”.
However, the operator said: “We will retain the four-hour journey time on our 0540 Edinburgh to London service. Reductions to journey times of other services will not be possible until the completion of further infrastructure work.”
A DfT spokesperson said: “Around £780 million of funding has been committed for investment in the East Coast main line.
“Once this programme is delivered, it will allow an enhanced timetable to be introduced on the route and a fastest journey between London and Edinburgh of around four hours using the Azuma fleet.”
However, it said reduced journey times would not come until the introduction of a new timetable in 2021.
Network Rail, which is responsible for the work, said the improvements, between Bawtry, south of Doncaster, and Edinburgh would be complete “by 2021”.
This includes upgrades to make sure there is enough power through overhead line equipment for the new trains. The southern part of the line was upgraded in 2017.
A spokesperson said: “We are working closely with our industry partners at LNER and [train manufacturers] Hitachi to make sure the full benefits of the pioneering Azuma fleet are delivered for customers as quickly as possible. We are currently carrying out a range of infrastructure upgrades to support the introduction of the new fleet and to cut journey times in the future.”
Anthony Smith chief executive of passenger watchdog Transport Focus said: “Passengers who have put up with slower services in the expectation of a better, more reliable and more comfortable rail service will be disappointed that promised improvements will be delayed.”