The investigation undertaken by The Guardian newspaper said on average customers paid for 12 megabits per second of broadband capacity.
However, on average, customers were only getting 7 megabits per second out of their broadband services, a difference of 42 percent, the newspaper said.
"If you were buying a dozen apples and you got three, you wouldn't put up with it," said Hugh Colvin, an arts organizer who pays for two phone lines to have enough Internet access for his family.
"It's outrageous that I pay the same as somebody who is in the middle of London getting 10 times the speed," he said.
The gap between promised speed and speed capacity delivered to homes varies from company to company, The Guardian said.
TalkTalk and Sky customers reported the largest gap with a 60 percent spread between advertised speed and actual speed. Virgin customers on average reported a gap of 41 percent between the promise and the product, the newspaper said.
BT customers fared better than the rest. They paid for an average of 8 Mbps and received an average of 6Mps, a difference of 27 percent.
"We have provided personalized speed quotes for some years now, so our customers know exactly what they're getting before any commitment is made," said BT, which no longer advertises a specific speed on its Web site.
"No one enters a contract with us without receiving a speed estimate tailored to them," a TalkTalk advertisement says.