MILK imports have rocketed by almost 60 per cent in just two years amid fears of a mass exodus of British farmers from the industry.
Farming leaders have blamed "ludicrously low prices" paid by supermarkets to producers for the collapse of the domestic market.
But with imports soaring while exports fall, Westcountry farmers are worried that in future, the UK could be "held to ransom" by other countries with a stronger dairy industry.
New figures show that last year 134 million litres of milk was imported into Britain, up from just 84 million litres in 2006. In the same period, exports fell from 511 million litres to 456 million.
Liberal Democrat environment spokesman Tim Farron said the surge in imports of liquid milk was "staggering" and warned more dairy farmers would be forced out of the industry unless action was taken by ministers.
"Britain is now importing record amounts of milk because our farmers are getting such a poor return for their efforts," he said. "This is a stupidly shortsighted attitude that will continue to undermine the UK dairy industry."
He said supermarkets would continue to import millions of litres of milk "without a second thought of the damage to our dairy industry because they know that will be able to cut their prices without cutting their profits".
Mr Farron said it was "about time" that the Government introduced the long-trailed supermarket ombudsman to give both shoppers and farmers a fair deal.
Defra has sought to blame "bad weather" for the sharp rise in imports.
Nationally, the number of dairy farmers has fallen by 10 per cent from 2006 to 2008.
But in the South West, there was a 22 per cent drop over the same period, to just 7,167.
Martin Whell, chairman of the South West dairy board, said three dairy farmers were now leaving the industry each day.
"If you look at the trend of how quickly farmers are getting out – because their cost of production is higher than what they are getting paid – the biggest concern is where is the supply going to come from in the next few years?"
An over-reliance on imports could see Britain "held to ransom" by other countries, he warned.
Mr Whell, who farms at Golant, near Fowey in Mid Cornwall, also called for a powerful ombudsman to "actually police how the supermarkets buy goods".
The Department for Environment, Food and Rural Affairs last night insisted that exports remained significantly higher than the level of imports.
"The UK produces and exports far more milk – in raw or liquid drinking form – than it imports," said a spokesman. "At certain times of year, there are natural drops in production and in 2008, bad weather exacerbated this, requiring a slight increase in imports in order to fulfil contracts.
"In 2008, the UK produced over 13 billion litres of milk, and the industry here is highly modernised, efficient, and well-placed to take advantage of the UK's productive capacity."