Royal Mail said on Saturday it was working hard to clear the estimated 60 million items of post that have built up during this month's strikes. An agreement in principle was reached between Royal Mail and union leaders on Friday to end the dispute over pay, working conditions and pensions.
Royal Mail said in a statement: "We are now working hard to return to normal as quickly as possible, but there is a significant build-up of mail that will take some time to clear.
"We will deal with all mail in the order in which we received it, and will re-instate all service guarantees and promises as soon as we are able to honour them." It said the time taken to clear the backlog would inevitably be lengthened while some staff still took unofficial strike action in several parts of the country.
On Friday, management and union leaders issued a joint statement that said only: "Agreed terms covering all the issues in the dispute will be considered by the union executive on Monday." Details of the agreement will only be released after it has been submitted to the union executive for approval on Monday.
The agreement in principle came hours after the Royal Mail won an injunction at London's High Court outlawing a new wave of postal strikes scheduled for next week. Recent 48-hour stoppages have caused delays and disruption, particularly to firms dependent on mail order business.
Prime Minister Gordon Brown had called on the postal union to end the strike, saying there was no justification for it. Royal Mail is fighting private competition from rivals after losing its 350-year monopoly on postal services last year. Its Chief Executive Adam Crozier says the company desperately needs to modernise to compete and that staff should be more flexible. Union chiefs say that some working practices at Royal Mail amount to slavery.