LONDON, Dec 6 (Reuters) - Workers at several Tesco (TSCO.L) distribution centres across Britain are set to strike in the run-up to Christmas, potentially hitting product availability, the Unite union said on Monday. It said warehouse and truck drivers based at depots in Belfast and Antrim in Northern Ireland, Didcot in southern England and Doncaster in northern England were taking strike action in protest over the supermarket group"s offer of a 4% pay rise.The Belfast and Antrim workers will be taking all-out strike action from Dec. 16. Didcot and Doncaster workers will strike for 48 hours from the same day and for five days from Dec. 20. Tesco, Britain"s biggest retailer, said it had made a fair pay offer."We welcome the decision by our colleagues at the sites who have voted against industrial action. We are disappointed that some have voted to proceed, and we have contingency plans in place to help mitigate any impacts," said a Tesco spokesperson. British retailers are already grappling with delays in international supply chains that are being compounded by labour shortages in domestic transport and warehousing networks, with a lack of heavy goods vehicle (HGV) drivers particularly acute. But Tesco said in October it was coping well. read more