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)
Virgin Media's 12 month plan was the only one that didn't impose a loyalty penalty when the initial term of the contract ended.
"Loyal broadband customers are being stung by big price rises once their fixed deal ends," Gillian Guy, chief executive of Citizens Advice, said.
"The government has rightly put energy firms on warning for how they treat loyal customers - the actions of broadband firms warrant similar scrutiny."
The Citizens Advice research also found that older people and poorer customers were more likely to be hit by such charges as they generally stayed with the same supplier for longer than other customers.
In the survey of 3,000 consumers, broadband users aged 65 and over were more than twice as likely as younger users to have been on the same contract for more than 10 years.
In March, an Ofcom report revealed that elderly people with a landline and no broadband at home had been hit the hardest by rising line rental charges.
BT announced price rises in January for many of its services, including regular and super-fast broadband.
Sky, Virgin Media and TalkTalk had also all put up their prices in the past 12 months.