Businesses in Wales are "crying out" for a Brexit plan, a Labour MP has warned the UK government in a debate.
Ogmore MP Chris Elmore said that he knows of a leading UK manufacturer with "serious" concerns.
He said the company, which wanted to remain anonymous, was already being hit by a downturn in construction and currency fluctuations.
UK ministers suggested MPs spend more time discussing Wales' "positive" jobs news, rather than "scaremongering".
Speaking in a Westminster Hall debate, Mr Elmore said that the firm had told him the scale of the potential change Brexit could bring about was "vast" .
He said that the company needed clarity about the form of Brexit, but that it specifically need to know if the government was going to keep the UK in the single market and the customs union.
"The bottom line is that business in Wales is crying out for a Brexit plan for Wales," he said.
The government's "vague platitudes" on what it wanted from the negotiations "serve only to distract from the fact that, as it stands, we're being led into the night without a torch", Mr Elmore added.
What is the single market?
- The EU single market is a free trade area, where there are no tariffs or taxes on trade between countries
- Its members trade freely with each other and impose common tariffs on imports from non-EU countries
- It also includes the free movement of goods, people and capital
And the customs union?
- In the EU customs union no customs duties are paid on goods moving between EU countries.
- All countries also apply a common customs tariff for goods imported from outside the EU.
- Goods legally imported into one country can circulate throughout the customs union with no further customs checks
Dwyfor Meirionydd Plaid Cymru MP Liz Saville Roberts said that there was uncertainty for rural firms about what would replace the Rural Development Programme and the Common Agricultural Policy after Brexit.
She gave the example of South Caernarfonshire Creameries, which she said generated at least £30m a year for the local economy, had expanded with EU monies, and was planning on expanding further.
"Since the vote to leave the European Union however, the government has failed to shed any light on how they intend to compensate for the millions of pounds lost," she said.
Wales Office Minister Guto Bebb said that Wales had performed extremely well from a jobs point of view and "every single measurement on employment has been positive".
He said the best performing part of the UK has been small businesses in Cardiff - with a 12 percent growth in turnover - "outpacing even London", something MPs should be talking more about that rather than "scaremongering", he said.
On Wales' involvement with the Brexit negotiations, there was an "ongoing engagement" from the Wales Office and across government, he added.