British farmers drove their tractors outside the UK parliament in central London on Monday to protest post-Brexit trade deals, which they say threaten food production.

Approximately 100 tractors from around the country converged in Westminster, where they blared their horns and displayed slogans including "Support British Farmers" and "Brexit is a disaster".

"We've had enough," organiser Liz Webster of Save British Farming told AFP, as she denounced accused "substandard" imports of "undercutting" British products.

The UK has signed several trade deals since it left the European Union in early 2020 after Britons voted to leave the bloc in a referendum four years earlier.

Farmers say some of the deals, and a lack of import checks, is allowing poorer quality food to come into Britain from countries with less stringent regulations.

They add that the policy in England of paying farmers to create habitats for environmental reasons was taking land out of food production.

Several demonstrations have taken place sporadically in recent months against the post-Brexit agricultural policy of the Conservative government, which has been in power for 14 years.

British agriculture provides around 60 per cent of the food consumed in the UK, but farmers fear this share is falling.

They also say they are suffering from rising costs and a shortage of seasonal workers, many of whom are foreign and whose recruitment is more difficult since Brexit.

Prime Minister Rishi Sunak promised to support UK farmers when he told the National Farming Union conference last month that "the nation's food security" was a "vital part of our national security".

The Tories lag well behind the main opposition Labour Party in opinion polls as a general election looms later this year.