Rail electrification projects on some routes across the south of England are running up to four years behind schedule, it has been reported.
Network Rail (NR), which is responsible for maintaining and developing railway tracks, admitted in October last year that the cost of electrifying the line between London and South Wales could reach £2.8 billion despite an estimate of just £874 million being given in January 2013.
The work was set to be finished by 2018 but NR has confirmed that the "scale and complexity" of the upgrade means some lines will be completed later than planned.
NR told the BBC that Bristol will not see electric trains until 2020 - four years later than the 2016 target - while Didcot has been pushed back two years to 2017.
Electrification at Newbury and Oxford will be three years late, with completion now due in 2018 and 2019 respectively, the report added.
An NR spokeswoman told the Press Association: "The Great Western main line to Cardiff - which forms the backbone of the electrification project and will carry the vast majority of services and passengers - will be electrified by 2018, the year originally envisaged.
"The scale and complexity of the electrification project, together with the other extensive upgrades we are carrying out, mean we will electrify other lines later than originally planned.
"Massive investment is taking place now and it will deliver faster, quieter and greener journeys for hundreds of thousands of passengers each year."