Issues with deliveries have caused turmoil for retailers. Almost 40pc more packages were returned between Christmas and New Year compared to last year, figures from returns company ReBound showed, after presents turned up too late.
Altogether the row with the Communication Workers Union (CWU) has cost Royal Mail around £200m since August, the company said on Thursday. Operating losses amounted to £295m in the first nine months of its financial year so far.
Unions have warned that more strike action could be on the cards, with the CWU balloting its more than 100,000 members, claiming that Royal Mail is staging “the most brutal attack on any group of workers the UK has seen in decades”.
Royal Mail has said it “would love to get back in talks, to get an agreement”.
Despite six more days of industrial action than first forecast, Royal Mail owner International Distributions Services said it was still set to meet guidance for annual operating losses of between £350m and £450m thanks to cost savings and strike contingency measures.
The group said it is still on track to cut its workforce by 5,000 by March and 10,000 in total by August. The number of voluntary redundancies needed will be far less than the 5,000 to 6,000 it initially expected thanks to employee turnover and cutting variable full-time staffing.
The threat of further strikes comes after Royal Mail was this month forced to stop international deliveries following a cyber attack, which has hammered smaller retailers who rely on shipping abroad. Some have warned the issues have pushed them closer to the brink, as customers waiting for orders overseas have been demanding refunds.
On Thursday Royal Mail said it was resuming its international deliveries, after two weeks of disruption.
International Distributions Services revealed that parcel volumes had also plunged by a fifth.