GSK employees to escalate strike action in June -union

View of GlaxoSmithKline headquarters in London

(Reuters) -Hundreds of GSK workers will this month escalate an ongoing strike over a pay dispute, UK labour union Unite said on Thursday.

Employees of the British drugmaker went on strike in May, having turned down the company's "significantly below inflation rate pay offer" of a 6% increase and a one-off lump sum of 1,300 pounds ($1,619.41).

The first strike this month will be held on June 9, Unite said, at all six GSK sites - Barnard Castle, Irvine, Montrose, Ware, Worthing and Ulverston.

Inflation has outstripped wage growth for most British workers this year, with labour unions representing nurses, teachers, transport staff and civil servants calling strikes to demand increases.

"We recognise that for many of our people, this past year has seen their cost of living rise rapidly and believe the offer we have made to our UK manufacturing colleagues covered by collective bargaining agreements is fair and reasonable," a spokesperson for GSK said in an emailed statement.

"We are therefore disappointed that the Unite union has chosen to undertake this industrial action, despite receiving a final offer," the spokesperson said, adding the company will continue to work to mitigate risk of significant supply disruption.

($1 = 0.8028 pounds)

(Reporting by Eva Mathews in Bengaluru; Editing by Nivedita Bhattacharjee and Dhanya Ann Thoppil, Kirsten Donovan)