Cameron Diaz and Jason Segel
Cameron Diaz and Jason Segel Reuters

Bad Teacher co-stars Cameron Diaz and Jason Segel have sparked relationship rumours after they were spotted having dinner in East Hampton, New York.

"They were at [an upscale eatery] Nick and Toni's and seemed to be on a date," a source told Life & Style magazine.

Another eyewitness reportedly saw the two grocery shopping together.

"They bought groceries and then drove off together," another insider revealed.

However, an eyewitness told E! Online that Diaz and Segel were joined by a third person at the restaurant and that they "were not romantic at all towards each other, very casual and friendly".

Segel, who has also been linked to Serbian-Australian actress Bojana Novakovic, broke up with actress Michelle Williams in February after dating for a year.

Diaz, 40, is currently busy filming her new movie The Other Woman, which also stars Leslie Mann, Taylor Kinney and Kate Upton.

Meanwhile, she will be seen alongside Segel in the upcoming film Sex Tape, where they play a married couple who accidentally misplace a sex tape they made of themselves. The 33-year-old How I Met Your Mother actor talked about his weight-loss regime, to prepare for the role.

"I've just been living healthy, exercising, eating right -- no more midnight pizzas," Segel told Us Weekly, "This time when I take my shirt off, I've made a promise to myself, it doesn't have to be funny."

"I took heat for Forgetting Sarah Marshall, where I was completely nude. But different characters require different body types. It is R-rated, and I'm not in shape for no reason," he added.

The movie is currently slated for a July 2014 release.