Staff at 70 Tesco stores will go on indefinite strike on Monday in a dispute over planned pay cuts for around 1,000 long-serving employees.
In 1996 Tesco introduced the so-called "modern" employment contract for new entrants which involved lower pay and conditions.
Pre-1996 employees were ring-fenced and allowed to retain their existing terms .
However, Tesco recently announced plans to transfer the 1,000 remaining pre-1996 workers to the modern contract - which Mandate says will mean significant loss of earnings for them.
That transfer to the new contract is due to take effect on Monday.
Some weeks ago, Tesco opened a redundancy scheme which has been accepted by 700 of the affected workers.
The remaining 300 have been offered a one-off compensation lump sum but their union Mandate has described that proposal as unacceptable.
Assistant General Secretary Gerry Light said that while the pickets would commence at the 70 stores where the pre-1996 workers are employed, Mandate is preparing to ballot members in all Tesco shops for strike action to support their colleagues.
Talks at the WRC last Monday failed to secure a resolution.
A spokesperson for Tesco said: "We're disappointed with Mandate's announcement that they will hold strike action on Monday.
"We remain committed to reaching agreement on this issue and earlier this week we formally tabled a generous proposal for compensation for colleagues in scope.
"We need to make this change in order to improve our customer service at our busiest times of the week.
"Our stores and online service will open for business on Monday as usual."
