Only 87 zero emission buses are in use in England outside London, figures published by Transport Secretary Mark Harper show.
In February 2020, then-prime minister Boris Johnson pledged to support the purchase of at least 4,000 new zero emission buses across the UK.
The commitment is for the end of the Parliament, which is widely expected to be in 2024 ahead of the next general election.
But in a letter to the Transport Select Committee, Mr Harper provided figures showing just 87 of the vehicles are on the road in England outside London.
Industry body the Confederation of Passenger Transport (CPT) called on the Government to explain “how it will ensure its commitment is delivered”.
Mr Harper’s statistics show a further 1,845 zero emission buses have either been ordered or are funded.
There is also funding for 880 of the vehicles across devolved administrations in Wales, Scotland and Northern Ireland. No information was provided on how many of these vehicles are in service.
Mr Harper stressed the figures were “not official statistics” and were “indicative”.
Transport for London’s website states there are “more than 950 zero emission buses” operating in the capital.
Mr Harper said: “Our commitment reflects that we want to work with all parts of the UK to decarbonise our transport system and transition to a zero emission bus fleet.
“This supports the UK bus manufacturing jobs right across the country including in Scotland, Northern Ireland and the north of England.”
He added: “We expect to announce an end sale date for diesel buses shortly.
“This will set out the last date by which it is possible to purchase a new diesel bus and will therefore further aid the introduction of zero emission vehicles to bus fleets.”
CPT director of policy Alison Edwards said: “Buses have a huge role to play in meeting our decarbonisation goals.
“The Government made a commitment through the National Bus Strategy to deliver 4,000 zero emission buses through the Zebra scheme.
“With just 87 of these on the road, we would like to see the Government outline how the remaining Zebra funding of £115 million will be allocated, and how they will ensure their commitment is delivered.”
Transport Committee chairman Iain Stewart MP said: “The figures we have seen give us reason to doubt that, at its current pace, the Government can reach its target of getting 4,000 zero emission buses on the road by the next election, although it says over 3,000 are in the pipeline.
“While we urge DfT to keep its foot on the pedal, our recent report on the Government’s National Bus Strategy argued that the real silver bullet to decarbonising this sector will be to improve the UK’s bus services generally, and encourage more people out of their cars, especially over short, local journeys.
“My Committee will keep a close eye on DfT’s progress as the deadline approaches.”