Image copyright Getty Images Brexit poses a huge risk to UK wildlife and habitats and a new environmental law will be needed, MPs have said.
Even where EU law is incorporated into the UK's legal framework there could still be problems, they warn.
The House of Commons Environmental Audit Committee also says farmers are facing significant threats including loss of income and tariffs on exports.
The government says it is committed to safeguarding and improving existing environmental protection.
Problems with incorporation
Whether it is the cleanliness of the UK's beaches, pollution control or the protection of species, much of the UK's environmental and wildlife legislation is rooted in EU directives.
The government has said __that it will introduce a "Great Repeal Bill" that will incorporate many of these regulations into UK law when Britain leaves the union.
However, Environment Secretary Andrea Leadsom admitted that about one third of the current environmental rules would be difficult to transpose into UK law, saying "there will be work to do to ensure we can continue to make these measures work once we leave the EU".
For instance, the EU's Birds and Habitats Directives have given much greater protection for wild birds and special areas of conservation than domestic UK legislation - but these will no longer apply in their current form in UK law even if Britain remains in the Single Market.
The cross party Environmental Audit Committee (EAC) says that to ensure that there is no weakening, a new environmental protection act should be passed during the Article 50 negotiations.
"European law protects huge amounts of the UK's environment, farming and countryside," Mary Creagh, chair of the committee, told BBC News.
"The process of leaving the EU presents a huge risk to all of those protections which is why in our report we're calling for new environmental protection act so that when we leave the European Union we are no worse off protected than we are at the moment."
And it suggests incorporating EU law into the UK legal framework could result in these protections becoming so-called "zombie" laws, which are no longer updated and could be easily eroded with minimal parliamentary scrutiny.
Environmental campaigners agreed that this was a significant concern.
"The Great Repeal Bill must contain democratic safeguards to make sure that no significant amendment to the scope or purpose of EU environmental legislation can be made without being subject to full parliamentary scrutiny," said Sam Lowe from Friends of the Earth.
"No one voted to 'take back control' for the UK Parliament, only to hand it straight over to a minister, brandishing a red pen, with the power to delete vital nature protections on a whim."
The report also focuses on the impact of Brexit on farming, pointing out that the Common Agricultural Policy (CAP) makes up between 50-60% of some farm incomes. The MPs say they have a number of worries about the future of farming.
"UK farming faces significant risks - from a loss of subsidies and tariffs on farm exports, to increased competition from countries with weaker food, animal welfare and environmental standards," said Mary Creagh.
"The government must not trade away these key protections as we leave the EU. It should also give clarity over any future farm subsidies."
Many environmental campaigners believe that Brexit represents a very good opportunity for reforming the EU farming policy that they say has been one of the biggest drivers of environmental decline in the UK and other parts of Europe.
"There needs to be substantial reform - if a farmer is holding water on his field upstream that is helping to prevent downstream flooding, that's not a commercial activity for a farmer but that's where public subsidy is warranted," said Trevor Hutchings from WWF-UK.
"There's a huge opportunity here to have a strong and thriving farming community, environment as well as servicing a public good."
Responding to the report the government pointed out that the UK is a signatory of international wildlife protection conventions, such as Ramsar and Bern, that are independent of EU membership.
A spokesperson said: "The UK has a long history of wildlife and environmental protection and we are committed to safeguarding and improving these, securing the best deal for Britain as we leave the EU."
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