Cycle hire firm Nextbike has come under fire after having stations full of bikes - but people can't use them.
The scheme works by people hiring out bikes across 81 stations in the Welsh capital via a mobile phone app.
However, a technical fault has beset the app for more than a week - meaning that there are racks full of bikes that can't be "unlocked".
The flaw is the latest problem for Nextbike, which recently revealed that more than three bikes are reported and vandalised every day .
Nextbike customers have branded the situation with the app as ridiculous - with many contacting the firm via social media to vent their frustration.
Jonathan Pollinger, 45, from Cardiff said: "There have been issues for about a week. There must have been about 15 bikes in the photo I posted on Twitter. Also, there were around the same number at Millennium Centre on Sunday and none of those were working.
"On both occasions I phoned them and they unlocked a bike for me although that was time consuming and painful."
Nextbike said the technical problems were a "global issue".
Krysia Solheim, Nextbike UK managing director, said: "Unfortunately we're experiencing technical problems with our rental systems due to a fault with our third-party mobile SIM network provider. This is a global issue, not just UK-specific one.
"The problem means that some rentals are not ending when bikes are being returned, and some bikes are not unlocking from stations. Our customer service lines are also experiencing higher-than-average call volumes because of the issue.
"We'd like to apologise for any inconvenience this has caused - we're hoping the issue will be resolved ASAP and we would like to thank everyone for their patience while we get this rectified.
"We're advising customers who experience problems returning bikes to contact us directly on Facebook or Twitter with details of the problem, including the bike number and the phone number, so we can sort out the problem asap. We would like to reassure our customers that they will not be charged for erroneous rentals during this period."
