Donald Trump has talked up the prospects of a "very big and exciting" post-Brexit trade deal between the US and UK, amid controversy over whether Britain would have to relax food standards to secure an agreement.
In an early-morning tweet, the US president hit out at the "protectionist" EU and said work was under way on what could be a "major" deal with the UK.
International Trade Secretary Liam Fox, who is in Washington for talks with US trade representatives, said that Brexit offered an "unprecedented opportunity" to reshape Britain's independent trading ambitions.
He played down critics' fears that British markets could be opened up to US agricultural products currently blocked by EU food standards rules, including controversial chlorine-washed chicken.
The comments came as a cross-party House of Lords EU committee warned that the Government's desire to secure free trade agreements after leaving the European Union could result in a "race to the bottom" on animal welfare standards as UK producers are forced to cut costs.
Mr Trump said on Monday that talks between Dr Fox and US Trade Representative Robert Lighthizer marked the opening of a "new chapter for stronger trade" which could make the trans-Atlantic special relationship "even better".
And on Tuesday morning, he followed up with a message: "Working on major Trade Deal with the United Kingdom. Could be very big & exciting. JOBS! The E.U. is very protectionist with the U.S. STOP!"
Former Ukip leader Nigel Farage said Mr Trump's comments showed there was "a big world for the UK outside the EU".
Following controversy over the US practice of washing poultry carcasses in chlorinated water to reduce the risk of contaminated meat, Labour accused Dr Fox of being prepared to "abandon British poultry farmers in favour of cheap US imports that do not meet our sanitary or animal welfare standards".
Campaigners against a "hard Brexit" challenged the International Trade Secretary to eat a chlorine-washed chicken on camera to prove he is happy that they meet the standards required by UK consumers.
But Dr Fox condemned media "obsession" with the issue, which he said would be only "a detail of the very end stage of one sector of a potential free trade agreement".
The House of Lords Energy and Environment sub-committee warned that animal welfare standards could be undermined if post-Brexit trade deals leave UK farmers competing against less-regulated foreign rivals.
The peers also raised concerns about the "overwhelming reliance" on EU citizens in crucial veterinary positions and urged ministers to make sure the industry was able to fill those roles after Brexit.
The report said: "Our evidence strongly suggests that the greatest threat to farm animal welfare standards post-Brexit would come from UK farmers competing against cheap, imported food from countries that produce to lower standards than the UK.
"Unless consumers are willing to pay for higher welfare products, UK farmers could become uncompetitive and welfare standards in the UK could come under pressure."
They warned that imports from lower-welfare countries could "undermine the sustainability of the industry or incentivise a race to the bottom for welfare standards - contrary to the wishes of the UK industry".
The peers also noted that much farm animal welfare research is backed by Brussels and "Brexit could lead to a major funding gap".
The sub-committee's chairman, Liberal Democrat Lord Teverson, said the Government "may find it hard to reconcile its free trade ambitions with its commendable desire for preserving high farm animal welfare standards".
At a breakfast meeting on Tuesday with members of Congress, Dr Fox outlined a UK report showing trading relationships between Britain and each of the 435 US Congressional districts.
A new US-UK Trade and Investment Working Group will seek to expand commercial links already worth more than £150 billion (200 billion US dollars) a year, he said.
"The EU itself estimates that 90% of global growth in the next decade will come from outside Europe, and I believe as the head of an international economic department that this is an exciting opportunity for the UK to work even more closely with our largest single trading partner the US," said Dr Fox.
He is also due to travel to Mexico to meet Mexican economy minister Ildefonso Guajardo to discuss the UK's post-Brexit trading relationship and then to Houston to hold meetings with the oil and gas industry representatives.
Liberal Democrat leader Sir Vince Cable called on the Government to guarantee parliament a vote on any post-Brexit trade deals.
"Liam Fox and Boris Johnson must not be able to stitch up trade deals abroad and impose them on the country," said Sir Vince.
"MPs must have the right to scrutinise and reject any deal that would be bad for UK consumers and farmers.
"It is Parliament, not Liam Fox, that should be the final arbiter on whether to sacrifice our standards to strike a deal with Trump."