A new £1bn fleet of South Western Railway trains faces a further delay.
Rail minister Chris Heaton-Harris said the 90 Arterio trains, originally due to start running in 2019, were now not expected to be in service until early summer.
The fleet is part of the operator's plans to provide more capacity and quicker journey times on its Berkshire, Surrey and south-west London routes.
South Western Railway said the delay was due to the impact of Covid-19.
Manufacturer Bombardier, which builds the trains in Derby, has previously blamed delays on software problems.
The trains, comprised of 750 carriages, had been expected first on the Reading and Windsor to Waterloo routes, which are normally among the busiest and most crowded in the country.
In a written reply to Slough MP and Shadow Transport Minister Tan Dhesi, Mr Heaton-Harris said: "This timeline is contingent upon there being no further delays in the delivery and acceptance of the new trains, a risk that is heightened with the current situation with Covid-19, and the completion of the associated training for the train crew to enable the trains to be operated in passenger service."
He added that a full roll-out of the new trains was expected in the 12 months after the predicted early summer launch.
In a statement, South Western Railway (SWR) said: "While we are sorry that Covid-19 has caused this delay, we remain more committed than ever to transforming travel on our Reading, Windsor and South West London suburban routes through this exciting £1 billion investment."
The current SWR fleets will be sent to work on the Southeastern Railway when the new trains come in.
Analysis: Paul Clifton, BBC South transport correspondent
The first Derby-built Class 701 Arterio should have entered service between Reading and London Waterloo in late 2019.
Through 2020, more promised deadlines came and went.
Now the Rail Minister says the first of 90 new trains should run in early summer, with the rest taking another year to trickle through.
Note his careful get-out-of-jail clause - "contingent upon there being no further delays in the delivery and acceptance of the new trains". There's no certainty here.
All previous versions of this Bombardier train have arrived late - Crossrail, London Overground and Greater Anglia had delays too. The manufacturer has previously blamed software issues.
I don't think either South Western Railway or the Department for Transport will be too upset.
These are ultra high capacity commuter trains, but passenger numbers this week are at 16% of pre-pandemic levels.
SWR simply doesn't need the extra carriages at the moment, and isn't generating fares to pay for them. Covid restrictions make driver training difficult too.