Quitting the European Union means the UK will have to go back to the drawing board over devolution, constitutional experts have warned.
Wrangles over sharing out cash for farmers will be one of the biggest challenges ministers face, according to the Institute for Government (IfG).
New agreements on how the environment, agriculture and fisheries are dealt with when powers return from Brussels are urgently needed, it said.
Jill Rutter, IfG Brexit programme director, said: “The past year has shown the strain leaving the EU is placing on devolution arrangements designed on the assumption of UK membership.
“It is time for an overhaul. It is in the interests not only of the UK government, but also the devolved governments, to develop firm foundations for future joint working – to promote collaboration and innovation.
“Only then will we have the right environment, agriculture and fisheries policies for the whole country after Brexit.”
The UK received £3.4 billion in Common Agricultural Policy (CAP) payments in 2016, with England allocated significantly less per person than the other nations.
According to the IfG, Northern Ireland received £177.64 per person, Wales and Scotland around £94 and England £41.46
As well as an agreement on how money should be allocated after Brexit, a decision must also be taken on what restrictions are placed on how the funding is spent, the IfG said.
Although some of it will fall under international rules, such as World Trade Organisation (WTO) regulations, they will not be enough to prevent market distortion within the UK.
“For example, current WTO rules would not prevent the Scottish government from substantially subsidising beef farmers, even if the UK government chose not to do so in England, which would give an advantage to Scottish beef farmers trading within the UK,” the report said.
The IfG called for an urgent review of the Joint Ministerial Committee, the forum for the UK and devolved governments to meet.
Westminster’s parliamentary committees must hold joint probes with Scotland, Wales and Northern Ireland in relevant areas, the IfG also recommended.
It said the creation of public bodies to replace EU institutions should be “four-nation by default”, including Environment Secretary Michael Gove’s plan for an environmental watchdog for England.
Martin Harper, RSPB’s director of global conservation, said: “Nature knows no borders. For very good reason the EU has set common standards on a broad range of environmental issues that affect us all.
“It is vital that our governments work together to agree new shared frameworks for nature’s recovery and new joint governance arrangements capable of holding them all to account.”
A UK Government spokesman said: “We have undertaken a great deal of work to strengthen our relationships with the devolved governments as we prepare to leave the EU.
“We want to ensure the Joint Ministerial Committee is as effective as possible for all members and have implemented several changes to improve its function.
“The Government recognises the need to review the existing intergovernmental structures and that is why at the most recent Joint Ministerial Committee the Prime Minister put forward a proposal to do just that.
“The UK Government will work in collaboration with the devolved administrations to review our structures and make sure they are fit for purpose in light of our exit from the EU.”