British Gas is to appeal against a tribunal decision which a union claimed could give thousands of workers holiday pay they have been denied.
Unison said an Employment Appeal Tribunal ruling on former British Gas worker Joe Lock meant employees would be able to have commission included in their holiday pay.
Mr Lock, who was employed by British Gas as a salesman, first took a tribunal case in 2012, complaining he was only paid basic pay when he took annual leave, with no payment for commission.
Unison general secretary Dave Prentis said: "This is a victory both for employees and for those that believe in fair pay.
"From the very beginning, Unison has fought for workers to have their commission included in their holiday pay.
"This case will have implications for thousands of workers across Europe who for years have been denied a fair deal.
"Until now some employees who rely upon commission and overtime have lost a significant amount of money. It's only fair that workers should receive their normal pay, including their regular commission, whenever they take their annual leave."
Unison said the case will have implications for workers who normally receive commission and potentially other similar payments and are paid less than their normal income when on annual leave.
Lucy Lindstrom, head of employment law at British Gas, said: "We note the decision of the Employment Appeal Tribunal. We have requested permission to appeal to the Court of Appeal so that there can be a definitive ruling on this issue.
"We are also continuing discussions already under way with our trade unions over how best to approach holiday pay in the future."