Tesco and M&S supplier G's Marketing is paying £220-260 per tonne for medium-sized brown onions - almost double last year, said commercial manager Tim Wigram.
"Retail prices haven't doubled, but they've gone up. More increases are on the way, but if prices get too high, we'll encourage a lot of imports. From February, we can buy from South Africa and South America, and New Zealand from April."
In a normal year, it costs £350 per tonne to source brown onions from New Zealand, so there's room for UK increases. "The UK has the best storage in Europe. We know we'll have the best quality - we just don't yet know how good that'll be."
Onion supply is estimated at 330,000 tonnes this year, down 55,000 tonnes on 2005 and short of the UK's 480,000 tonne requirement.