Analysis of the cheapest broadband deals from the five largest suppliers by Citizens Advice found bills soared by more than 40% on average at the end of the fixed contract period.
Researchers found more than a third of broadband customers were unaware they could face additional costs when staying with a provider when their deal ends.
Gillian Guy, chief executive of Citizens Advice, called on the Government to subject broadband firms to the same level of scrutiny as energy providers.
"Loyal broadband customers are being stung by big price rises once their fixed deal ends," she said.
"People often choose their broadband deals based on the price that works for them - but our evidence shows that many do not realise the price will rise after the end of the fixed deal.
"With people staying with their supplier for an average of four years, these extra costs can run into hundreds of pounds."
Ms Guy said extra protection for elderly and vulnerable customers is "a must".
The charity compared the cheapest basic broadband deals from BT, Virgin Media, Talk Talk, Sky and EE.
The deals from BT, Virgin and Sky lasted 12 months, EE's was 18 months and Talk Talk's was 24 months in length.
Researchers found the BT deal rose by 67%, the Sky deal increased by 53%, the EE deal went up by 36% and the Talk Talk deal rose by 28%.
There was no increase when the Virgin Media deal ended.
On average the bills rose by 43%, or £9.45 more a month, adding £113 a year.
The charity also surveyed more than 3,000 broadband customers and found they stayed on the same contract for an average of four years.
Pensioners were more than twice as likely than customers under 65 to have been on the same contract for more than 10 years.
Similarly those on low incomes were almost three times more likely than high earners to remain in a contract for 10 years or more.
Ms Guy said: "Older customers and those who have less money are more likely to stay with their supplier for longer meaning their loyalty penalty could reach over a thousand pounds.
"The Government has rightly put energy firms on warning for how they treat loyal customers - the actions of broadband firms warrant similar scrutiny. Extra protections for vulnerable consumers are also a must."