Brexit trade negotiations and Covid-19 are "two perfect storms", a Welsh Government minister has said.
Lesley Griffiths, who is responsible for rural affairs, said "the window of opportunity to secure a deal is getting very, very small".
The UK is due to leave the EU without a trade deal unless an agreement can be found before the end of the year.
Welsh Tory politician Darren Millar accused the Welsh Government of not moving on from lost battles.
Both the UK government and the European Union are due to decide by the end of this month whether the current deadline for negotiating an agreement should be extended beyond the end of December.
The UK has said it wants to reach an agreement but will not agree to any extension, even if the EU requests one.
On Thursday, business leaders warned that a no-deal Brexit would be catastrophic for British firms.
Dame Carolyn Fairbairn, from industry body the CBI, told the BBC that any buffers to cope with the additional cost and planning of an exit from the EU customs union and single market without a deal had been exhausted by the Covid-19 pandemic.
Earlier this month, a UK cabinet minister said a deal was still possible with the EU after both sides admitted little progress had been made.
Talks broke up last week, with sticking points including future fishing rights in UK waters and commitments to maintain a "level playing field" over regulation and competition.
Giving evidence to the Welsh Parliament's Climate Change, Environment and Rural Affairs Committee, Lesley Griffiths said: "It's two perfect storms I would say, and I think it's incredibly concerning that the UK Government are remaining adamant that we will be leaving.
"I don't think the Welsh Government believe it's doable."
She added: "We're now in the middle of June, by the end of July the big trade negotiations should have been closed. The threat of a no-deal Brexit is increasing."
"It's the same officials that are dealing with EU transitions and we've had to deal with Covid-19. So I don't think nightmare is too strong a word. It is incredibly difficult to do everything that we need to do but of course we are trying to manage that".
'Not moving on'
But Welsh Conservative Brexit spokesman Darren Millar said the Welsh Government was "clearly not moving on from the battle they've already lost about Brexit".
"The will of the Welsh people was that we should leave the European Union, the UK government has now taken us out and it set a very clear timetable which businesses are expecting to be met about the negotiation of a free trade deal.
"The very last thing that we ought to be doing frankly is extending that period. We need a deal as soon as possible.
"Taking our foot off the pedal in anyway is going to potentially thwart that."
Wales' Brexit Minister Jeremy Miles said it would be "irresponsible" for the UK government not to seek an extension to current the transition period.
Speaking to BBC Wales, the Welsh Government politician added: "What the British government should be doing is to seek an extension, give space to negotiate and give organisations and businesses right across the UK time to prepare."