Scottish and Welsh environment ministers are to meet to co-ordinate opposition to Brexit legislation.
The two administrations fear the Westminster government is planning to take over powers which are currently devolved, but exercised in Brussels.
Scottish Environment Secretary Roseanna Cunningham said a UK-wide framework for green targets could hold Scotland back.
UK ministers have insisted that any changes after Brexit will enhance, not reduce, environmental protection.
Environment Secretary Michael Gove has targeted a "green Brexit" which would strengthen current rules safeguarding nature wherever possible.
The Scottish and Welsh governments have banded together to oppose moves from Westminster on several occasions in recent months, with first ministers Nicola Sturgeon and Carwyn Jones issuing a joint statement describing the EU Withdrawal Bill as a "naked power grab".
Ms Cunningham's visit to Cardiff to meet Welsh counterpart Lesley Griffiths comes three weeks after Scottish Brexit minister Mike Russell made a similar trip to sit down with Welsh Finance Secretary Mark Drakeford.
They maintained their position that they could not recommend that Holyrood or the Welsh Assembly give legislative consent for the withdrawal bill, which is designed to translate EU law onto the UK statute book as part of the Brexit transition.
Ms Cunningham contended that about 80% of Scots environmental law originated at EU level, including legislation around waste management, pollution, biodiversity, flooding and drinking water quality.
The MSP said imposing a UK-wide framework could potentially undermine the "more ambitious" targets set in Scotland.
She said: "Imposing a UK-wide framework for the environment risks undermining the significant progress Scotland has made, which has seen us win international recognition for our work on climate change and the circular economy.
"We are not opposed in principle to UK-wide frameworks in certain areas but this must be through agreement - not imposition.
"Protecting devolution will allow us to drive forward our ambitious work to reduce greenhouse gas emissions, enhance environmental standards and create a cleaner, greener Scotland for everyone."
Ms Griffiths added: "Devolution has enabled the Welsh government to deliver ground-breaking legislation for the people of Wales, which delivers on international obligations and has been recognised as cutting-edge by a number of international institutions.
"The approach presented by the UK government in the Withdrawal Bill could significantly undermine this progress."
The UK government has insisted that the devolved administrations will actually see an increase in responsibilities post-Brexit, with Scottish Secretary David Mundell referring to it as a "power bonanza" and First Secretary of State Damian Green saying that "there is absolutely the opposite of the intention of a power grab".
Mr Gove meanwhile has claimed that Brexit plans could enhance environmental protection, saying the UK could be a "global leader in environmental policy" outside of the EU.
He said there could be changes to the operation of some rules, but said any change would be "designed to ensure we get better protection for the environment".
He said: "I have no intention of weakening the environmental protections that we have put in place while in the European Union.
"Informed by rigorous scientific analysis, we can develop global gold standard policies on pesticides and chemicals, habitat management and biodiversity, animal welfare and biosecurity, soil protection and river management and indeed in many other areas. We can take smarter and more targeted approaches to the improvements that we want to see."
Talks between Scottish and UK ministers in Edinburgh ended in stalemate earlier in August, with further discussions of devolved powers to be held in the coming weeks.