"Red Tractor doesn't differentiate us," he said. "It doesn't tell the consumer anything more than the Sainsbury's name or a Union Flag."
He continued: "Why would we lend credibility to a label that anyone can use? We are low users of certification schemes that have a low price of entry. They can be the refuge of scoundrels who don't want to cover the hard yards of differentiating themselves."
Describing King's remarks as "frustrating", NFU vice president Meurig Raymond said: "We in farming are proud of Red Tractor."
King went on to say that 35 per cent of Sainsbury's tomatoes and cucumbers are now sourced from UK suppliers, up from 25 per cent four years ago, and it has also begun sourcing UK-grown cranberries and figs.
But he added: "Doubling UK sourcing doesn't mean we will stock 100 per cent British. We start with the consumer and work back. People want asparagus and lamb year-round, but even though 35 per cent of our asparagus is British, the season is 12 weeks long at best."
While acknowledging the growth on online retail, he said: "In 10 years' time, over 80 per cent of supermarket shopping will still be done in familiar large stores."
Challenging times George Eustice on agricultural work and a changing labour pool
Asked whether Defra would consider a replacement for the Seasonal Agricultural Worker Scheme, abolished last year, farming minister George Eustice told the NFU Conference:
"I understand the industry's concerns, having worked in the fruit industry in Cornwall for 10 years. But then there were 15 member states, but now there is a much greater pool of labour. Today's challenges are not the same as 20 years ago. We have to make it easier for local people to take jobs in farming - we shouldn't tolerate the attitude that 'they won't do it'."
Outgoing NFU president Peter Kendall said: "The government tells people 'do it', but this doesn't mean they will. The nation will pay the price for this social experiment of getting (local) people to work on the land. Everything that farmers complain about is hardwired into the Common Agricultural Policy through EU regulations set centrally in Brussels."