Sainsbury and Morrisons have been targeted by shopworkers' union Usdaw for a recruitment campaign.
Usdaw broke through the 100,000 members barrier at Tesco last week and has plans to grow membership.
Both Sainsbury and Morrisons claim to be happy for Usdaw to increase its influence in it stores. Colin Moffat, HR business partner at Sainsbury, said: "We have a very proactive relationship with Usdaw and recognise that by working together we can balance the needs of the business and the needs and interests of colleagues."
Morrisons has an existing partnership agreement with Usdaw which it claims adds value to the business.
Usdaw has a partnership agreement with Tesco which it says provides "excellent recruitment facilities". This includes inviting the union to inductions when new stores are opened.
The partnership agreement is under review with both parties committed to strengthening the relationship. "The relationship is positive and constructive. It improves consultation and gives our staff a voice in the company," said a Tesco spokesman.
The union now has 318,000 members, up 10,000 in the past year.

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