Theresa May refused to guarantee she will not water down food standards or open up the NHS to US firms in a trade deal with Donald Trump.
The Prime Minister faced repeated questions about how much she is prepared to give away, ahead of her face-to-face talks with the President later this week.
Jeremy Corbyn urged her to rule out any deal that would give US healthcare giants a toehold in the NHS – after similar concerns over an aborted EU-US agreement.
And the SNP raised fears that such a deal will open the door to British supermarkets being stocked with meat produced in unhygienic ways currently outlawed across the EU.
The price of freer transatlantic trade will include the sale of chickens washed with chemicals – a practice in the US – critics say.
In further evidence of growing worries at Westminster over a headlong rush to get close to Mr Trump, a Tory MP demanded a guarantee the UK will not “facilitate torture”.
And Ed Miliband, the former Labour leader, urged her to take along British scientists who could convince the President that climate change is “not a hoax invented by the Chinese”.
In reply, Ms May said her government was “very clear we don’t sanction torture” – and made clear she hoped Mr Trump will not walk away from the Paris agreement to cut carbon emissions.
However, she declined to discuss details of her hopes for trade from her trip to Washington, instead saying they were to “increase prosperity and bring growth”.
Mr Corbyn warned of “a blank cheque to President Trump”, telling MPs: “Many have concerns that, in your forthcoming meeting with President Trump, you will be prepared to offer up, for sacrifice, the opportunity of American companies to come in and take over parts of our NHS or our public services.
“Will you assure the House that, in any trade deal, none of those things will be offered up as a bargaining chip?”
Angus Robertson, the SNP leader at Westminster, said: “They want to export genetically modified organisms, beef raised with growth hormones and chicken meat washed with chlorinated water.
“Will the Prime Minister tell President Trump that she is not prepared to lower our food and safety standards?”
But Ms May, in reply to the Labour leader, said her early meeting was evidence of the bond between the two countries, “a special relationship on which he and I intend to build”.
And, she told Mr Robertson: “It is very simple – we want to achieve an arrangement that ensures the interests of the United Kingdom are there and are put first.”
She did add: “I can ensure the right honourable gentleman that, in doing that, we will put UK interests and UK values first.”
Nick Clegg said Joe Biden, the former US Vice President, told him the US would not agree to “anything that the chicken farmers of Delaware don’t like”.
Those farmers use chemical washes – to make up for inadequate hygiene at farms and abattoirs, food experts protest.
In contrast, the EU uses a so-called “farm to fork” approach, requiring steps all along the production chain to ensure the food sold is safe.