Chiltern Railway: Passengers warned of disruption
Passengers travelling on Chiltern Railway's route between Birmingham and London face "significant disruption".
The line is going to be closed in various locations from Saturday until 27 August.
The firm said it had been carrying out upgrading work, including new track, for some time but needed a longer closure period to finish the work.
It could not be completed without the partial closures, a spokesman said.
The company said it had been building new track, overseeing bridge replacements and the installation of new signals and switches for some time as part of its Chiltern Mainline project.
The project aims to make sure its trains can run faster.
The final block of work will see the track realigned at Neasden Junction and new signals installed along the line.