GMB previously said the workers would strike on Jan. 20, Jan. 22, Jan. 25, Jan. 29 and Feb. 1 over what it called plans to "fire and rehire" and cuts to workers' pay and terms. On Monday, it said the strike would be expanded to include Jan. 30 and Sun 31.

Workers already carried out a five day strike against the proposals at the beginning of January.

(Reporting by Susanna Twidale; Editing by Edmund Blair)