Train carriages running during peak morning and evening times have been cut by a third - from 12 to eight carriages.
A total of six services are affected - two in the morning and four in the evening - each offering 277 less seats than normal.
That’s a reduction of 1,662 seats per day and means commuter misery for workers.
London Midland has apologised to customers and says the shortage is due to a new fleet of trains not being up to scratch.
A spokesman for the firm said: “On September 20, we introduced a different fleet of trains – the Class 319 model – into our fleet. One of the trains we have leased arrived in a poor state of repair and needs a number of essential maintenance works carrying out.
“Our engineering team is working hard to repair the faults and as soon as this is complete, we will bring the train into operational service.
“We apologise for the inconvenience this is causing to your journey”.
Commuters are being advised to travel on earlier or later unaffected trains if possible.
When trains are particularly busy, the conductor does have the authority to declassify the First Class section.
The times when shorter trains are running are; the 7.21am train from Milton Keynes Central to London, the 7.39am train from Bletchley to Euston, the 5.41pm train from Euston to Tring, the 6.05pm train from Euston to Northampton, the 6.27pm train from Tring to Euston and the 7.24pm train from Euston to Milton Keynes Central.
At this stage, London Midland is unable to say how long it will take to repair the Class 319 train.