The Welsh Government is calling for changes to key Brexit laws in order to avoid a "major constitutional battle".
Finance Secretary Mark Drakeford said ministers in Westminster must bring forward "firm proposals" in amendments to the EU Withdrawal Bill.
The Welsh and Scottish Governments have called the proposed law a "power grab".
Ahead of talks in London, Mr Drakeford welcomed progress in EU negotiations but said a lot of work was needed to "achieve the right sort of Brexit".
First Minister Carwyn Jones had recently said he was "encouraged" to hear UK ministers concede that changes will need to be made to the legislation.
But changes proposed by the Welsh and Scottish Governments that were debated in the Commons last week were rejected by MPs.
Ministers in Cardiff and Edinburgh are concerned that, under the current plans, powers in devolved matters are set to transfer from Brussels to Westminster for an unspecified period of time.
The UK government says EU functions in areas like agriculture should initially be held at Westminster to allow UK-wide frameworks to be developed.
Speaking before a meeting of the Joint Ministerial Committee (JMC) on Brexit in London on Tuesday, Mr Drakeford said: "While we are clear that a UK-wide approach that respects devolution is the best approach, we are ready and able to put in place our own legislation.
"Work on this has been underway for many months and everything will be in position, should it be needed."
The Welsh Government has prepared a so-called continuity bill as a way of keeping EU regulations in Welsh law through assembly legislation.
Last Friday, Prime Minister Theresa May reached agreement with EU officials to move the Brexit negotiations on from separation issues - citizens' rights, the Irish border, and the UK's 'divorce bill' - to discuss the future trading relationship.
Mr Drakeford said it was "vital" that the devolved administrations are involved in the next phase of the Brexit talks.
"These negotiations will involve questions such as agriculture support during and after the transition period, the sort of environmental regulation we will have in future and whether the UK will continue to be part of important European programmes for education and research, such as Erasmus Plus and Horizon 2020 - all issues which are firmly within the remit of the devolved institutions," he said.
"It's essential that the UK government agrees a common approach to these and many other issues - not least our future access to the single market - with the devolved administrations in advance of the negotiations, based on a proper consideration of the evidence."
The meeting of the JMC will be chaired by the prime minister's deputy, First Secretary Damian Green, with the Welsh, Scottish and Northern Ireland secretaries also attending.