The acknowledgment came in response to a new report by sustainable transport campaigners Transform Scotland, which predicted only 16 per cent of buses would be electric or hydrogen powered by then.
To the surprise and scepticism of the bus industry, last year’s SNP Holyrood election manifesto stated: “We will remove the majority of fossil fuel buses from transport in Scotland by 2023.”
The “very ambitious” target was also included in ministers’ Programme for Government a year ago, which required more than 1,850 of Scotland’s 3,700 licensed buses to become zero emission, according to Transform Scotland’s report.
The total number of zero emission buses has increased from 272 to around 280 since then, with funding for another 325, which would take the total to just over 600, the campaigners said.
The Stuck in Traffic report stated: “With only approximately 16 months left to remove the majority of diesel buses from public service, it is unlikely that this target will be reached”, which it described as “not particularly surprising”.
Transport Scotland blamed the pandemic, but said it had been “right to aim high”.
Marie Ferdelman, the report’s author, said: "To miss the bus decarbonisation target would be hugely disappointing.
“There has been good progress on increasing the number of zero-emission buses on Scottish roads, but the Scottish Government should be judged on whether they can deliver on what they have promised and, in this case, they will not be able to do so.
"Efforts will have to be redoubled to transition Scotland's bus fleet to zero-emission vehicles.
The report also found “slow or insufficient progress” on free bus travel for under-22s, bus priority measures, rail decarbonisation, low emission ferries and car traffic reduction.
It said bus priority on the Glasgow motorway network had failed to be delivered and the public transport “Fair Fares” had still to be completed.
However, it said the promised increase in the active travel budget [for walking, cycling and wheeling] and the phasing out of petrol and diesel vehicles remained on track.
The Scottish Liberal Democrats said ministers must now say when their bus target would be reached and the remaining diesel buses phased out.
Climate crisis spokesperson Liam McArthur said: “Scotland needs to be pressing onwards in building a sustainable and eco-friendly public transport network, so it’s disappointing the Scottish Government’s bus target is being missed so dramatically.
“Buses are the backbone of the public transport system, so electrification would allow huge numbers of passengers to travel in an environmentally friendly manner.”
Transport Scotland did not dispute the report’s claim the bus target would be missed.
Its spokesperson said: “We’ve always been clear this was an ambitious target that we could not meet alone.
“That’s why the work of our bus decarbonisation taskforce has been critical to our success so far.
"Additionally, no-one could have anticipated the profound impact of the global pandemic across the globe let alone [on] the bus sector and these targets.
“We were right to aim high and to ensure progress across the sector, with the proportion of zero emission buses in Scotland now approximately three times higher than that in England.”