Some broadband customers who move to BT's ADSL2+ service could find that they experience a slower connection than the one they had before.
According to PC Pro, some of the unpredictable elements of the service mean that BT will aim to stabilise the connection for some people, which could result in it slowing down.
BT told the site that for the most part, however, consumers will notice an improvement.
The operator said that each line is provided with the "fastest, most reliable service".
However, a company spokesperson explained to PC Pro that it is more important to have a stable line in some cases than a fast one.
The spokesperson observed: "There may be issues over the speeds of a minority of lines that previously had not experienced issues with an up to 8Mb product when migrated to ADSL2+.
"BT aims to minimise this by identifying customers that may fall into this category, and migrating them onto ADSL2+ products that deliver a comparable performance to fixed rate and IPStream Max products."