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James Middleton, Donna Air

David M. Benett/Getty Images

Kate Middleton and Pippa Middleton's younger brother is back on the market.

James Middleton, 28, and girlfriend Donna Air, 36, have called it quits, E! News has learned. 

They did not comment, although Air posted on Wednesday a cryptic picture on her Instagram page, showing a blackboard with an illustration of a person lying on a yoga mat, with an inspirational quote written on it.

James and Air, a British model and TV host who has a 12-year-old daughter from a previous relationship, had been dating for more than two years and had occasionally attended celebrity events together. They were last photographed at the 2015 Wimbledon tennis championships in London in July.

A photo posted by Donna Air (@airdonna1) on

Donna Air, James Middleton

Karwai Tang/WireImage

'Unfortunately this split has been on the cards for months, since Donna accompanied the Middletons to St. Barts," a source told E! News, referring to the family's vacation in late August. "Donna desperately wanted make it work but James has been distant for a long time."

"Sadly the age gap did put pressure on the relationship, for although she already has a daughter, James did always feel like she perhaps wanted to start a family, and that is not where he is at all," the source said. "Donna is devastated, she's putting on a brave face, but this is a massive blow."

Another source told E! News Air never really fit in with the Middletons and never "gelled" with his mother, Carole Middleton, 60, or sister Pippa, 32.

Donna Air, James Middleton, Pippa Middleton

David M. Benett/Getty Images

"Donna is very sensitive and took this quite personally," the source said. "Of course, the Middletons were friendly to her, but Donna always felt like she wasn't totally part of the family. She and James had very, very different upbringings."

"There's no one else involved at this stage, but James is hot property, so no doubt there will queues of eligible women throwing themselves at him!" the source added.