EU officials are growing increasingly alarmed at the prospect of a “wild west” Brexit outcome in which a legal vacuum prevents the UK from punishing companies that flout environmental laws.

The petrochemicals company Ineos has threatened to close its Middlesbrough plant with the loss of 350 jobs unless it is allowed to defer compliance with EU rules on water and air pollution, according to documents obtained by Greenpeace.

In all, the EU is the source of up to 80% of the UK’s environmental laws, covering chemicals pollution, plant emissions, pesticides, landfills, recycling, climate change and more.

But the monitoring and enforcement powers of the EU commission and European court of justice will end with Brexit, potentially leaving a gap until a weaker UK replacement body becomes operational in 2021.

The environment secretary, Michael Gove, admitted last week that environmental protections in a no-deal scenario were currently “suboptimal”.

In the event of no deal – which some EU sources now see as the most likely outcome– legal experts predict a “governance chasm”, with potentially severe environmental consequences.

One EU official said the threat of a breakdown in regulatory order across the UK was “a real issue” that had not been fully grasped.

He told the Guardian: “The concern could indeed happen – that suddenly at the borders of the EU you have a country applying sub-standards on environmental legislation or social legislation or not abiding by its own national framework and we are not able to do anything with it. That is not so good for the environment.”

“If we are in a total wild west with no deal, they will be hit with WTO tariffs and checks,” he said, adding that chemical companies which did not respect EU rules “will not get on the EU market”.

Unlike the European commission’s environment department, the UK’s proposed office for environmental protection would not be launched as a fully independent body, and would lack powers to fine or punish environmental violations.

The Department for the Environment, Food and Rural Affairs (Defra) is setting up a smaller interim secretariat to bridge the gap until its debut, employing just 16 officials – compared with 445 in the EU’s environment directorate.

Some UK lawbreakers could be taken to court in the UK under the interim procedures, Gove told an environmental audit committee on 26 March.

But enforcement plans would probably be overwhelmed in any no-deal departure from the EU. This remains the default outcome if agreement is not reached by 12 April.

A no-deal outcome would add huge workloads to UK agencies already stretched thin, as they tried to replicate the work of EU institutions amid overlapping demands on the chemicals sector, and tariffs and barriers in clean energy trade.

Tom West, a legal adviser to the green law firm Client Earth, said: “Our statute books would look deceptively similar but they wouldn’t function effectively as they would have less capacity and resources. They would have lost access to all EU institutions and, crucially, they would have lost enforcement powers.”

“There is also a risk of the UK coming under pressure to strike new trade agreements quickly so as to be able to access goods and services we would need. These may also put downwards pressure on standards.”

However, such moves would conflict with the promise from the former Brexit secretary David Davis that Brexit would not plunge Britain into a “Mad Max-style world borrowed from dystopian fiction”, according to Tim Durrant, a researcher at the Institute for Government.

He said: “The government is in a minority in the House of Commons and we have seen how difficult it is for them to get their own business through, so it is very unlikely they will introduce massively deregulating legislation when there would be little support for that.”

A Defra spokesperson said: “Whether we leave the EU with or without a deal, our clear vision for a Green Brexit will mean environmental standards are not only maintained but enhanced.”