Two departments are being “left in the dark” about how to prepare for Brexit and lack a “clear plan” of priorities, a committee of MPs has warned.
The environment and trade departments have been hampered by “pervasive uncertainty” about the UK’s future relationship with the EU, they said.
Defra must drop or postpone some non-EU projects if it is to deliver its EU Exit programme, it suggests.
The government said Whitehall was “rising to the challenge” of Brexit.
In its report, the public accounts committee noted the “unprecedented challenge” facing Defra and the Department for International Trade.
Defra is among the departments most affected by Brexit, dealing with 64 areas related to it, including import controls on animals and animal products, to the authorisation of new chemical products.
The committee says both departments are “optimistic” about being ready by the time the UK exits the European Union, in March 2019, whatever the outcome of Brexit negotiations.
But it adds: “Their preparations, however, are being hampered by the pervasive uncertainty about the UK’s future relationship with the EU, which leaves not only departments but also businesses in the dark about exactly what they need to do to prepare.”
The committee says it is “concerned about how realistic the departments’ plans for Brexit are” – especially those which require new IT systems.
“Both departments have an impossible challenge and don’t have a clear plan of top priorities,” the report says.
And it says while they may feel the post-Brexit transition period agreed with the EU gives them “breathing space”, it did not mean “they can take their foot off the gas”.
Defra in particular cannot do everything it is currently doing and must set out a list of programmes it plans to stop, postpone or minimise by the end of July, it says.
The public accounts committee says it is “unrealistic” to expect Defra to make its planned £138m of savings in 2018-19, while also delivering its Brexit programme and doing its usual work.
Much of the department’s Brexit-related work involves new IT systems which the department has a “poor track record” on, the report says.
If these are not ready, the department says it could resort to “manual workarounds” – which the committee says would be “costly and embarrassing”.
“There are substantial risks, including disruption to the agri-food and chemical industries, if Defra’s IT systems are not ready in time,” it said.
It is also having to prepare for different scenarios based on whether the UK reaches a deal with the EU, or exits with no deal – which is “time consuming and costly”.
On the Department for International Trade, the committee questions whether it has sufficient expertise for trade negotiations and suggests it did not have “adequate understanding” of the regional impacts of Brexit on jobs and trade.
Committee chair Meg Hillier said departments were “under extreme pressure”.
“Our committee has repeatedly raised concerns about government’s preparedness for life outside the EU. The clock is ticking and there is still no clarity about what Brexit will mean in practice”, she said.
A spokesman for the government said: “We have already agreed the terms of an implementation period that will provide businesses with the continuity they need to prepare and thrive after we leave the EU.
“Work is being undertaken across the whole of government, in a range of exit scenarios in preparation for our withdrawal from the EU.
“Close collaboration between departments is vital as we negotiate our exit from the EU and develop our future trade policy with the world, and Whitehall is rising to the challenge.”