Citymapper, one of the most popular city navigation apps, has confirmed that it is experiencing problems with its service.

The London-based start-up, launched in London in the summer of 2012 before expanding to seven other cities, including New York and Paris, said on Twitter: “Experiencing some issues, investigating, sorry, pls stay tuned.”

Users are currently unable to search for routes on the iOS and Android mobile apps while the website is failing to load at all. 

The service, said to be installed on half of the iPhones in London, appears to have gone offline between 6am and 8am this morning and was still out of action at the time of writing.

Thousands of commuters rely on the navigation app to get to and from work each day, with many of them taking to Twitter to express their anger. 

) Tweeted:" down the first day of work. Not lucky" while holly catford (‏@hollycatford) Tweeted: “Citymapper please come back, I'm lost!”

The cause of the outage, which appears to be limited to London, is currently unknown but one Twitter user asked Citymapper if it was down to server issues. They did not get a reply. 

Having recently raised $10 million from investors, Citymapper is commonly thought of as one of London’s fastest growing start-ups.

Citymapper failed to respond to Techworld before publication.

Update: At approximately 11am, Citymapper Tweeted: "Everything is back to normal. Sorry again. Our server engineer just got back from his sunny honeymoon and is back in rainy action."