Uncertainty around Brexit could prevent banks from lending to long-term projects in Wales, an expert has said.
Graeme Yorston, the former chief executive of Principality Building Society, warned investment in businesses could stall as banks go into "risk averse mode".
HM Treasury said it would strengthen "financial services relationships with non EU countries".
The Welsh Government said it has been working closely with businesses.
Mr Yorston, who was chief executive of Principality Building Society for five years, said if banks try to save costs due to lack of clarity about their future, they could defer long-term decisions, such as investing in businesses to help them grow or set up.
A recent report in the Financial Times suggested Brexit would push up costs for banks by as much as 4% and their capital requirements will rise up to 30%.
He told BBC Radio Wales' Good Morning Wales programme two key areas would need to be looked at - the regulation of UK banks and building societies and the impact on the UK domestic market.
"I think the whole uncertainty and the lack of answers... undoubtedly mean that banks and indeed building societies tend to think about things perhaps with a little bit more of a risk averse perspective," he said.
"By that I mean they might have to take some longer-term decisions that they would've taken with a degree of certainty, against the backdrop of uncertainty they may delay those decisions.
"Whether that be investing in their own business to make them grow, or whether it be lending to UK-based businesses who may have a market in the EU."
He said banks would focus on the things that they can control, such as costs, which he believes could have an impact on branches across Wales.
Mr Yorston said the Welsh Government was aware of the effect this might have on small and medium enterprises.
But he added banks and building societies may have seen Europe as "being over-regulated" and might welcome "getting back control" of financial services regulation.
A HM Treasury spokesman said: "The UK economy is fundamentally strong but we are not complacent.
"We will strengthen our trade and financial services relationships with non EU countries to create significant value for the banking industry, and ensure an environment in which firms can operate at the point of Brexit.
"We are determined to give businesses, banks and households the certainty they need to help us make a success of Brexit and its opportunities."
A Welsh Government spokeswoman said it had been "clear that in the negotiations to leave the European Union we want the best possible outcome for the people of Wales".
She added: "We have been working closely with businesses of all sizes and from all sectors since the Brexit referendum. This has included meetings with businesses and representative bodies such as the FSB, CBI, IOD and the Chamber of Commerce as well as extensive engagement with SMEs.
"Through this work we have identified some of the key issues facing companies, the potential opportunities and challenges associated with Brexit and the areas where additional interventions or tools may be needed."
The spokeswoman pointed out that the launch of a Development Bank of Wales from October would "support small, medium and micro sized businesses get access to the finance they need to support and grow their businesses".