Ten farms in Scotland are still under restrictions because of radioactive contamination from the Chernobyl disaster exactly 20 years ago.
Some upland sheep are still being tested
The Food Standards Agency said it may be several years before the farms in Stirlingshire and East Ayrshire can be given the all-clear to sell stock.
About 3,500 sheep are currently being monitored but farmers' leaders have called for a new risk assessment.
A plume of radioactivity blew across Europe after the accident in 1986.
Heavy rain soon afterwards washed some of the contamination onto parts of upland Scotland.
Radioactivity clung to the peaty soil and was taken up by grazing sheep.
Initial movement and sales restrictions on almost 3,000 farms were quickly lifted.
Sandy Moffat has one of the remaining affected farms in East Ayrshire.
He told BBC Scotland: "When the restrictions were announced it was pretty horrific, there were about 2,900 farms in Scotland that were affected.
"Obviously we were very alarmed as to whether there would be any long-term future for farming in Scotland."
The restrictions were lifted by the end of 1986 but were reimposed on 73 farms when contamination returned in the next crop of lambs.
It has been a slow process clearing the entire Scottish flock, as some contamination remained.
Stuart Purdie, an agricultural officer with the Scottish Executive's rural affairs department, carries out testing on lambs.
He said: "We take a background reading using the monitor against one of ourselves, find the background reading for the day, there's a factor then added onto that for a pass limit so that we know whether the animals are above or below the accepted level."
The FSA also ensures that contaminated food does not reach the table.
Sandy McDougall, the agency's head of contaminants, said there may be some animals in Scotland which approach being up to two-times the safety limit.
Chernobyl, where the worst nuclear disaster happened 20 years ago
But he said: "All of the sheep-meat leaving such restricted farms is monitored to ensure it is well below the safety limit."
The process involves gathering sheep once a year for testing and getting subsequent tests before animals can be sent for slaughter.
The National Farmers Union Scotland said it was time to revisit the science and review the situation.
Borders hill farmer Bob Howett, the union's vice-president, was unaffected by Chernobyl.
But he said: "Maybe 20 years on it's time for the Scottish Executive and others to have a look and maybe do a little bit of work to see why are we left with this pocket of farms, where one or two drop off every year.
"Maybe it's time to take an assessment of what the risk is now."
The FSA said it reviewed the situation regularly, but there were too many factors involved to determine accurately when the restrictions would be lifted.
So far, more than £3m in compensation has been paid to farmers in Scotland.