Warnings have been given of "serious consequences" if old trains are not allowed to continue running on the Welsh network.
From 2020 trains need to be accessible for people with reduced mobility.
The UK government needs to give permission for non-compliant trains to continue running - but it has not been granted yet for Transport for Wales.
Ministers have urged UK counterparts not to allow the general election to get in the way.
New Porterbrook-owned Class 769 trains, which meet with disability laws, were due to have been delivered to train operating company Transport for Wales (TfW) last spring. But they have been delayed until early 2020.
Plaid Cymru AM Rhun ap Iorwerth said that if permission was not given, the removal of non-compliant Pacers from service would mean losing "nearly half the rolling stock used for Valleys lines".
Several other trains could also be grounded, leading to services potentially axed in "Fishguard, Wrexham, Holyhead, in order to maximise the rolling stock available for use on the Valleys lines", he said.
"There are serious consequences to not getting that dispensation," he added during a debate on the issue in the Welsh Assembly.
He blamed the Welsh Government, saying it did "not act quickly enough to seek a dispensation to continue using non-compliant trains after the end of this year when it became clear that new trains wouldn't be available on time".
Tory economy spokesman Russell George accused ministers of not taking appropriate action to address the issues with capacity on Wales-only rail services as far back as 2006.
But Labour AM Hefin David said he had been told by Transport for Wales they expect the Department for Transport to give the go ahead by the end of November.
Transport Minister Ken Skates said the Welsh Government had recognised the risks of ageing rolling stock and requirements for people with restricted mobility legislation.
"For the Wales and Borders operation, the shortage of units was identified in 2017 and as a result, trains were ordered then from Porterbrook for introduction in May of 2018," he said.
"But those trains have not been delivered as Porterbrook promised, and the programme is now almost two years late."
"The only option available was to apply for dispensation."
A Welsh Government motion, passed in the debate by a majority of AMs, called for the UK government to ensure the general election does not interfere with the request from the Welsh Government for permission.
The Department for Transport has said that dispensations can be given during a pre-election period.
Brexit Party AM David Rowlands said his group supported the Welsh Government's approach to the UK government.
The fears of disruption come after Transport for Wales announced an additional 186 Sunday train services.
The train operating company said Sunday services would be "transformed" from December by a 40% increase.