Tesco faced the High Court today over a controversial fire and rehire policy that union workers claim will leave workers thousands of pounds out of pocket.
Union Usdaw argues the grocer is acting "unconscionably" by trying to "unilaterally remove" workers' entitlement to retained pay.
Lawyers told judges that Tesco was proposing to issue termination letters with new proposals on working terms that would not protect their pay.
They claim workers will lose up to a third of their income by signing new contracts that will affect working hours, benefits and extra pay.
Union bosses want Mrs Justice Ellenbogen to "restrain" Tesco.
Lawyers representing Tesco are disputing Usdaw's claim.
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© Getty ImagesThe Union claims at least a third of workers could be worse off under the changes
They say Tesco is using a "contractual mechanism" open to employers.
Usdaw had announced in February that it was taking High Court action.
A spokesman at the time said that around 74 workers at three Tesco distribution centres in England - two in Daventry Northamptonshire, and one in Lichfield, Staffordshire - were at risk of having their "terms slashed through a fire and rehire process", which would cost some of them "a third of their wages".
He said "fire and rehire" was a "controversial process" where employers fired employees before rehiring them immediately afterwards, on less favourable terms and conditions.
A barrister leading Usdaw's legal team on Wednesday told Mrs Justice Ellenbogen said Tesco was acting "unconscionably".
In a written case outline, Paul Gilroy QC said: "The claimants' position is that given the assurances to affected staff (analogous to contractual undertakings) the defendant now acts unconscionably in seeking to unilaterally remove entitlement to retained pay.
"Unless restrained, the defendant proposes to achieve this by issuing notices of termination and offering re-engagement on new terms which do not include any retained pay."
He said Usdaw wanted "declaratory and injunctive relief" to restrain Tesco "from doing this".
The cases continues until Thursday with an outcome expected in the coming weeks.