NSA said farmers had bought lambs with the expectation of a change to a fixed cut off date.
Industry chiefs have urged Defra and the Food Standards Authority to push forward and finalise sheep splitting rule changes before 2019 lamb store sales begin, after confirmation regulations will not change this season.
In an official letter to industry stakeholders, Defra confirmed it would not be introducing an alternative for aging lambs at slaughter this season.
Last November, industry chiefs celebrated news changes to the rules to move to a fixed cut of date would be implemented in time for the 2019 season. But in February, it was revealed this would not be the case.
In February, the delay was attributed to the UK having to negotiate its status as a third country once it leaves the EU.
And Defra has concluded it will not be feasible to deliver the change in time for this year’s proposed cut-off date of June 30.
However, it said it would continue to plan for introduction and wanted to work with the industry, ’bearing in mind the background of the UK’s planned exit from the EU’.
The National Sheep Association (NSA) said it was disappointed but it was pleased producers now had certainty this season.
NSA chief executive Phil Stocker said he was disappointed another season would pass at a cost to the sheep industry estimated to be over £24 million a year.
“There will be some farmers out there who have bought lambs and fed them with the expectation of the move to the June 30 date, and the only silver lining is that people now know what they are working with for the remainder of this season and any uncertainty is now past.”
NSA was now pushing for a timetable to ensure this change is properly worked on and implemented ahead of the 2020 season.
Mr Stocker added: “While the change will not come this season there is no reason why Defra and FSA should not continue to push forward immediately with the aim of finalising things before the 2019 store lamb sales begin in late summer.
Richard Findlay, NFU livestock board chairman, said: “We had hoped if there was an agreement about Brexit for March 29 they had agreed to look to see if they could introduce this for possibly May and June.”
But with Brexit not resolved he said it was no surprise.
“The NFU and industry generally will work to agree a timetable for the introduction by October. To deliver this in time for lambs next season, that are being born this season.”
He added mouthing will have cost the industry this year, particularly as we move towards June.
“It is an extremely unusual situation where it has been announced by a Minister and Government agencies have not been able to deliver it,” he said.
“People at the end of last year bought stores in the belief mouthing was a thing of the past. They have been let down badly.”
NFU Scotland livestock committee chairman Jimmy Ireland said: “The official announcement from DEFRA that it plans to delay the implementation of changes to sheep ageing for the purposes of TSE controls is no surprise, given its statement in March, but continues to be a source frustration.
“Farmers and crofters in Scotland want to move away from the practice of checking teeth to a system which provides greater certainty for those marketing lambs.
“The small bit of good news is that the Scottish Government has undertaken the necessary steps to implement the regulation required to enable a change in this area, but we need to see the UK Government move to introduce this at a UK level.
“While the change has been delayed, NFU Scotland will be seeking an agreement on how to administer a change so that it can be implemented as soon as reasonably possible.
“This is yet another area where Brexit uncertainty is having an effect on enabling positive change for businesses across Scotland.”