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 or not Britain would have to relax food standards to secure an agreement.

In a 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.

Liam Fox, the International Trade Secretary, who is in Washington for talks with US trade representatives, insisted Brexit offered an "unprecedented opportunity" to reshape Britain's independent trading ambitions.

The Scot played down critics' fears that British markets could be opened up to American 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 the UK Government's desire to secure free trade agreements after leaving the Brussels bloc could result in a "race to the bottom" on animal welfare standards as UK producers are forced to cut costs.

Mr Trump said talks between Dr Fox and Robert Lighthizer, his US counterpart, marked the opening of a "new chapter for stronger trade," which could make the trans-Atlantic special relationship "even better".

The President tweeted: "Working on major Trade Deal with the United Kingdom. Could be very big & exciting. JOBS! The EU is very protectionist with the US. STOP!"

Nigel Farage, the former Ukip leader, said Mr Trump's comments showed there was "a big world for the UK outside the EU".

But Sir Vince Cable, the Liberal Democrat leader, claimed any potential US-UK trade deal was "not remotely comparable to the potential losses" Britain would sustain if it withdrew from the EU through a hard Brexit.

And the London MP called on the Tory Government to guarantee Westminster 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," declared 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," he added.

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 Secretary of State to eat a chlorine-washed chicken on camera to prove he was happy that they met 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 has warned that animal welfare standards could be undermined if post-Brexit trade deals leave UK farmers competing against less-regulated foreign rivals.

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 would seek to expand commercial links already worth more than £150 billion a year, he explained.

"The EU itself estimates that 90 per cent 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."

The Trade Secretary is due today to travel to Mexico to meet Ildefonso Guajardo, the Mexican economy minister, to discuss the UK's post-Brexit trading relationship and will then go on to Houston to hold meetings with oil and gas industry representatives.