Mila Kunis and Ashton Kutcher 'secretly engaged'

and

have secretly been engaged since the summer, according to new reports.

, who has been dating his girlfriend since spring 2012, is said to have popped the question on Mila's birthday back in August."It happened over the summer," an insider told

."Ashton's whole family had come out to California for Mila's birthday [on Aug. 14], and he did it then. They couldn't go public with [the engagement] until his divorce was final. Now nothing should be standing in their way."

© Photo: Twitter

Ashton, 35, split from Demi Moore, 51, after six years of marriage. Their

last week, two years after the couple had announced their separation. It is thought that the paperwork was delayed by disagreements about finances.Since the divorce was settled,

has been pictured wearing a ring on her engagement finger, fuelling rumours that the loved-up couple are set to tie the knot.A fan who bumped into Ashton and Mila in a yoghurt shop in Los Angeles told

that the 30-year-old actress was definitely sporting a ring on her finger."She was wearing a small ring on her left hand," said the source. "It was petite, but still noticeable and pretty."Mila looked really gorgeous and happy. They came into the store hand-in-hand and smiling. They looked like a cute couple."

© Photo: Rex

Speculation of the couple's engagement first started in September when the

actress was spotted wearing a simple gold band.Ashton and Mila first met on the set of

, where they played on-screen couple Jackie and Kelso for eight years. While Ashton went on to marry Demi, Mila dated child star Macaulay Culkin from 2002 to 2010.