The cheapest broadband prices shoot up by an average of 43% or £113 a year, after introductory deals end, Citizens Advice has said.
The charity said more than a third of customers were unaware of the price increases.
The rises amount to a "loyalty penalty" for customers who stay with the same provider, Citizens Advice said.
It has urged broadband providers to be more transparent about prices and said government should scrutinise the firms.
The £113 figure represents a five-fold rise on what customers were paying on average in 2011 to stay on the same broadband deal.
Four of the five biggest internet service providers had "loyalty penalties" as follows, according to Citizens Advice:
BT 12 month contract: £198 (67% increase)
Sky 12 months: £120 (53% increase)
EE 18 months: £90 (36% increase)
TalkTalk 24 months: £66 (28% increase)
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