Trade deals after Brexit could pose the "biggest peacetime threat" to the UK's food security if standards and farmers are not protected, it has been warned.
The Government must ensure future trade deals protect British farmers, and do not undermine them by allowing imports of food produced with lower welfare or environmental standards.
The prospect of deals with countries including the US have raised concerns over lower-standard food, such as chlorine-washed chicken and hormone-fed beef, entering the UK and making it hard for farmers to compete with the cheaper imports.
And free trade deals could lead to direct foreign investment, which could encourage farmers to adopt more intensive farming systems that move the UK away from sustainable ways of producing food and managing land, the All-Party Parliamentary Group on Agroecology, a cross-party group of MPs and peers, warned.
A report released to coincide with the Oxford Real Farming Conference, taking place at the same time as the Oxford Farming Conference in the city, warned most farmers outside the east of England would struggle to compete at world market prices.
And with 60% of food, feed and drink exports going to the EU, the most important trade arrangement for the UK to resolve was with the bloc, it said.
The group's chairwoman, Labour MP Kerry McCarthy, said: "There are serious concerns that if negotiators don't value farmers enough and build poorly managed trade deals that reflect this - particularly a US-UK deal - it could trigger a race to the bottom in terms of standards and ability of our own farmers to compete.
"The APPG is determined this sector should not become a bargaining chip or something that can easily be traded."
Environment Secretary Michael Gove, who is speaking at both the farming conferences in Oxford on Thursday, has insisted the UK will not weaken environmental or animal welfare standards to secure a trade deal with the US.
He has also indicated farmers will be paid for delivering benefits for nature and the countryside after Brexit instead of receiving subsidies for the amount of land they farm.
But International Trade Secretary Liam Fox has played down concerns over issues such as chlorine-washed chicken.
In his New Year's message, National Farmers' Union president Meurig Raymond called for a post-Brexit policy that allowed farms to be profitable, productive and progressive.
Mr Raymond said opportunities would arise throughout the Brexit negotiations and "we must be ready to take them wherever we can".
A Defra spokesman said: "We are committed to securing the best possible trade deal when we leave the EU - one which includes a comprehensive free trade deal with the European Union and, in due course, the chance to negotiate new free trade agreements with other countries."