Royal Mail's parent firm, International Distributions Services, on Wednesday offered a pay increase of up to 9% over 18 months in its "best and final" offer to its largest labour union, the Communication Workers Union (CWU).

Royal Mail said it will make Sunday working optional and offer "more generous" voluntary redundancy terms than originally proposed, while also committing to no compulsory layoffs until the end of March.

The revised offer comes after months of pay dispute between Royal Mail and the CWU. The union has engaged in strike action over multiple days in the past few months and recently announced another 10 days of strikes between Nov. 24 and Dec. 24, a busy period for the postal firm.

The CWU, which represents more than 115,000 postal workers at Royal Mail, last month rejected a 7% salary increase over two years.

The union did not immediately respond to a Reuters request for comment.

(Reporting by Aby Jose Koilparambil in Bengaluru; Editing by Anil D'Silva)