A total of £30m should be spent protecting firms from possible damage from Brexit over the next two years, Plaid Cymru's leader has said.
Leanne Wood, who will address Plaid's annual conference in Caernarfon later, said the Brexit Preparedness Fund could help "mitigate any Brexit shock".
Mainly small to medium-sized firms (SMEs) should get specialised help and financial assistance, she said.
Plaid have already agreed a £5m scheme in a budget deal with Labour ministers.
Ms Wood warned that if UK-EU trade talks failed, and Welsh ministers had "no plan" to deal with the consequences, "Welsh nationhood" was at risk.
A "range of measures" were needed to help protect Welsh businesses from any negative impact from leaving the European Union, she said.
"One such measure is a Brexit Preparedness Fund designed to help businesses evaluate their exposure to Brexit and then receive specialist advice and financial assistance," she said.
"Such a fund should be targeted mainly at SMEs and should be approached proactively and engaged with on an all-Wales basis."
Ms Wood said Plaid Cymru would suggest "constructive proposals the Welsh Government should adopt to protect our economy and constitution" as well as applying "relentless scrutiny of the UK government's actions and their impact on Wales".
In an interview with BBC Radio Wales, Ms Wood said the final outcome of the negotiations should be ratified by either a referendum or a vote of AMs.
She told the Good Morning Wales programme: "Whatever deal is finally agreed, whether it is a deal or it isn't a deal, I think it should be ratified either by people in a referendum or by people's representatives in the Welsh Parliament.
"If there is a deal that is going to be damaging and risky then people deserve to have a say and endorse or not that deal."
The Rhondda AM has insisted she will lead the party at the next assembly election in 2021 and that she has the backing of the membership.
It follows reports of disquiet among some Plaid Cymru AMs over whether she should lead the party for another four years.
Plaid lost 14 deposits and its vote share fell in June's general election, but it won an extra parliamentary seat meaning it now has four MPs at Westminster.