ROME (Reuters) - Leaders of Italy's Fiom union are due to meet on Wednesday and are likely to call a strike against a labor deal that car maker Fiat FIA.MI clinched with other unions last week, a Fiom source said on Tuesday.

The deal, signed by all Fiat unions except the left-leaning Fiom, is part of efforts by Fiat Chief Executive Sergio Marchionne to boost productivity at plants in Italy by making labor practices more flexible.

In return, Fiat has agreed to a 1.3 billion euro ($1.71 billion) investment programme at its Mirafiori plant in Turin.

Fiom, which has bitterly opposed the deal, wants to hold the strike at the end of January, the trade union source told Reuters on Tuesday. A formal announcement is expected on Wednesday.

A workers’ referendum on the deal is likely to take place in January.