Which? is calling on providers to give their customers "the speed and service they pay for" after a study found 63% of consumers experience problems with their broadband and 45% suffer slow download speeds.
Of the 45% experiencing slow speeds, 58% said this was a frequent or constant problem.
Some 36% of people have had intermittent connection drop outs and 15% have on occasion not been able to connect at all.
The survey also found households are experiencing poor customer service, with 27% waiting two days after reporting a loss of service to have it resolved and 11% waiting a week or more.
The watchdog found 31% who contacted their provider with a problem did not get a resolution at all, one-quarter of those who got a resolution were dissatisfied with how long it took, and one-fifth had to contact their provider three times or more to fix the issue.
Which? is campaigning for providers to give consumers written speed estimates at the start of the contract, allow customers to leave contracts without penalty if they do not get that speed and fix any loss of connection quickly, with refunds for loss of service.