About 135 staff at manufacturing firm Fenner Dunlop are facing shorter working hours for the second time in months.
The company, which makes conveyor belts for the mining industry, put half its employees on a three-day week in December.
They were able to return to full hours earlier this summer but have now been told they could be cut again.
Political instability in Eastern Europe because of the Ukraine crisis has led to a market downturn, bosses said.
Manufacturing manager David Wood said: "We've announced there's a possibility of short-time working.
"We have issues with the ongoing conflict in Ukraine and the impact on exchange rates.
"Medium to long-term, we still see everything being OK. We're not looking at any redundancies."
The last slowdown was caused by a reduction in demand from coal and potash mines across the world.
It meant Canadian customers built up a surplus of machinery made at the factory in Marfleet Lane, east Hull.
However, staff came back to full hours after the stockpile ran out and more equipment was needed.
Demand is rising again from companies producing potash, which is used to make fertilisers.
But with many of Fenner's key coal customers affected by the Ukraine crisis, that market continues to lag behind.
Mr Wood said: "We've seen a small return to normality on the potash side but the coal market still seems a concern, especially with Russia and Ukraine.
"We have a few customers over there.
"We see a slow return to normality with potash.
"That market is returning to normality, not as quickly as we would like, but we see some signs of improvement."
Staff were told of the possibility of short-term working at a 10am meeting on Friday.
They were given four weeks' notice, meaning it will not be introduced before mid-October at the earliest.
Mr Wood said it was too soon to know how many staff would be asked to come in and stressed nothing was set in stone.
He said: "We're not definitely going into a short-time period – we just have to give notice there's a possibility.
"We're not in one yet. Hopefully we won't have to go into one."
The company was confident things were improving only a fortnight ago.
At the time, human resources manager Diane Quigley was optimistic about the future.
She said: There are some difficulties because we do supply into the Ukraine, specifically into the area where the troubles are currently taking place, but other markets are holding up.
"We do still have some issues and we're keeping the situation under review.
"Longer-term, we see the outlook as good."
The factory's parent business Fenner is listed on the stock exchange and is one of Hull's most successful companies
It released a mixed trading statement in July, saying the Advanced Engineered Products division was performing well.
However, Fenner said the Engineered Conveyor Solutions arm, which includes the Hull factory, was suffering due to decreased interest from the US coal and Australian mining markets.
'At this stage, the warning is precautionary'Many Fenner Dunlop employees are represented by GMB.
There has been no official contact on the possible reduced hours between the groups as yet, but full-time union officer Dave Oglesby is looking to find out more as soon as possible.
He said: "They've not formally written to me."There's a four-week lead-in to reduced hours.
"Anything could happen in four weeks – at this stage, the warning is precautionary.
"I'm looking to meet them within the next week or so. I had a conversation with our rep and we're looking to have a meeting with them as soon as possible."
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