The Communication Workers Union (CWU) said progress had been made in discussions, but more time was needed.
A joint statement by the union and Royal Mail said: “Negotiations under a new process between the senior leadership of both Royal Mail Group and the CWU facilitated by Brendan Barber (former TUC general secretary and ACAS chair) supported by Marina Glasgow (chief conciliator of ACAS) have been taking place.
“Talks continued this weekend. Progress has been made in some areas and it has been agreed to extend the talks into this week.
“Both parties reiterate the importance of the full adherence to the joint statement agreed on March 2 and the need for workplace tensions to be reduced.”