Shares in Moneysupermarket fell nearly 8% after the comparison website warned that revenues so far this year were running below last year's levels.
The FTSE 250 company said this was due to customers switching less between savings and current accounts because of low interest rates.
The news overshadowed the 16% rise in annual profits to £73.5m that the company reported for 2016.
Another big faller in the FTSE 250 was transport group Go-Ahead.
Its shares dived nearly 14% after the company warned of lower-than-expected full-year profits, partly due to the disruption caused by strikes on Southern rail services.
Go-Ahead owns a 65% stake in Govia Thameslink Railway (GTR), which runs the Southern rail contract.
Half-year profits at Go-Ahead fell 11.7% to £67m, and the company said its results had been "significantly impacted by a reduction in rail profitability due to losses from our GTR franchise, as a result of ongoing industrial action on Southern".
The benchmark FTSE 100 index was little changed in early trade, up just 0.75 points at 7,253.75.
On the currency markets, the pound was unchanged against the US dollar at $1.2440, and down 0.1% against the euro at 1.1740 euros.