Royal Mail downgraded its letter volume guidance, blaming the impact of new European privacy regulations and 'business uncertainty'.
For the nine months through December, the company said its overall trading performance was broadly in line with expectations.
However, in the letters business, volumes were down 8%, while revenue fell 6%.
Addressed letter volume declines, excluding elections, were expected to fall 7-8% in the year through March.
'While the rate of e-substitution remains in line with our expectations, business uncertainty is impacting letter volumes,' chief executive Rico Back said.
'As a result, addressed letter volume declines, excluding elections, are likely to be outside our forecast medium-term range next year.'
'Otherwise, we are reconfirming the outlook and other guidance for 2018-19 provided in our half year results.'
Royal Mail confirmed that it expected to deliver a full-year operating profit before transformation costs of £500m-to-£530m.
In the parcels business, volumes and revenue both rose 6%.