Royal Mail could cut as many as 5,000 to 6,000 jobs by the end of August next year and warned of more layoffs if planned strikes go ahead, as the British company remains locked in a bitter dispute with its largest labor union.

The Communication and Workers Union (CWU), representing 115,000 Royal Mail postal workers, have held strikes in September and early October, and have threatened more strikes this month and next.

"Royal Mail urges CWU to immediately call off planned strike action and embrace our offer of Acas talks to urgently find a resolution to the current dispute," parent company International Distributions Services said on Friday.

Royal Mail's proposals last month to take the pay dispute to arbitration and change its policies following months of failed negotiations had angered the CWU.

Royal Mail saw adjusted operating losses of 219 million pounds ($247 million) in the first half of the year, including about 70 million pounds from the three days of strikes in the period, and losses for the year is expected to be around 350 million pounds, it forecast.

That could increase to 450 million pounds in losses this year if customers, who switched providers during the strikes, continue to do so for longer periods, it added.

"We will be starting the process of consulting on rightsizing the business in response to the impact of industrial action, delays in delivering agreed productivity improvements and lower parcel volumes," it said in a statement.

The company said there could be more job losses at Royal Mail, its U.K. business, if strikes went ahead during the holiday period.

"This announcement is holding postal workers to ransom for taking legal industrial action against a business approach that is not in the interests of workers, customers or the future of Royal Mail," CWU General Secretary Dave Ward said.

"The CWU is calling for an urgent meeting with the Board and will put forward an alternative business plan at that meeting."

GLS, its international distribution network, has been performing in line with expectations, the company said.

Royal Mail, the former British postal monopoly, recently changed the name of its holding company to International Distributions Services from Royal Mail Plc.