Jemima, 40, was seen wearing a diamond ring yesterday in east London and she apparently looked very happy, reported Daily Mirror.
"It looked like an engagement ring. It was very pretty and classy. She looked very happy," the source said.
The 38-year-old funnyman, who fuelled dating rumours with Jemima last year, had earlier said that the journalist made him a "changed man".
Jemina has two sons with former cricketer Imran Khan. She also dated actor Hugh Grant.
The 'Get Him To The Greek' star has remained tight-lipped about his love life since splitting from Katy Perry in December 2011 after 14 months of marriage.