The founder of Foursquare has been forced to issue a public apology on behalf of his wife, who ran the Boston Marathon using another runner's official number.

Foursquare founder Dennis Crowley and his wife, Chelsa Crowley, ran the marathon together earlier this week after attempting to complete it together in 2013.

It was only days later another marathon runner accused Crowley of running the marathon as a 'bandit' - someone who doesn't officially register to run or pay any fees.

Runner Kathy Brown told WCVB in Boston that she registered through the official channels and was running for a charity, meaning she had earned her real bib number on the day.

After completing the race, Kathy logged onto a photo website to see the pictures taken of her running when she saw the fake bib on Chelsa.

She told the TV station:

"I opened it up looking for the pictures of me and there were some in there and they came out really good, but then I saw this other woman.

"It was pretty easy to figure out that someone just made a fake bib."

Brown noticed Chelsa had put her Twitter handle on the bib, and had tweeted about running as a bandit.

She was angered by Chelsa's disregard for her own efforts and training and made a point of telling reporters:

"I put that work in and it wasn't fair that someone else didn't have to do that. That was my first thought."

Foursquare founder Dennis suffered online backlash and was forced to write an apology on his website.

Attempting to explain his wife's actions, Dennis wrote that the couple were running the marathon together in 2013 but were separated before the bombs went off. While Chelsa made it across the finish line moments before the first bomb exploded, Dennis was unable to get to the finish line, so he was allowed to run again automatically in 2014 but she was not.

In the post, Dennis wrote:

"Yes, using a duplicate number to get Chelsa into the starting corral with me was wrong. I don't expect everyone to understand our strong need to run and finish together - but after trying unsuccessfully to get a charity number and trying unsuccessfully to officially transfer a number from an injured-runner friend, we did what we could to make sure we could run together in hopes of finishing together."

"I sent an email to Kathy Brown, the woman who rightfully earned #34033 to apologize for any disrespect, hurt feelings or confusion. Our intent was never to 'steal' anything from anyone - our intent was to finish the Boston Marathon together as we tried to do last year."

Boston Marathon officials are now looking into multiple accounts of bandit runners from this year's race.