Royal Mail has said it is keeping a close eye on the UK's economic growth amid fears of a sharp slowdown following the Brexit vote.
The group said that changes in gross domestic product (GDP) were key "drivers" for its letter and parcel business and confirmed it was "monitoring the situation".
Royal Mail, which was fully privatised last October, posted a first-quarter trading update showing further falls in letter postings, with volumes down by 2% in the three months to June 26 - or a decline of around 4% with EU referendum mailings and direct delivery returns stripped out.
Revenues from letters were 3% lower in the quarter, but overall group turnover was 1% higher in a "quieter trading period", according to Royal Mail.
Its parcel business helped offset some of the ongoing decline in letters, with volumes and revenues both 2% higher, thanks to an "improving trend" in demand from consumers and small businesses.
Chief executive Moya Greene said: "In what is traditionally a quieter trading period for the business, we saw no material change in overall trends.
"We continue to face the challenges caused by the current low inflationary environment and our highly competitive markets."
Royal Mail's European parcels business, GLS, which accounts for almost a sixth of group revenues, saw revenues and deliveries both rise 13%, boosted by the timing of Easter and other public holidays in Europe.
Rob Byde, analyst at Cantor Fitzgerald, said it was a "cautious" trading statement from Royal Mail.
But he said Royal Mail should weather any wider woes in the economy following the EU referendum vote.