Jennifer Aniston claims she had no idea her fiance Justin Theroux was going to propose.

The 'We're The Millers' actress - whose 18-carat engagement ring came under fire last week with some critics calling it ''too big'' and ''gaudy'' - has revealed the surprise proposal was a ''very covert operation'' and she had no idea he was planning to pop the question on his 41st birthday in August.

She told People magazine: ''It's hard to pull one over on me, where I'm not like, 'Hmm, you're acting really strange.' It was ... a very covert operation.''

The 43-year-old actress also hit back at critics for suggesting her engagement ring is too big and insisted she loves it despite not selecting it herself.

She said: ''I was impressed. I had no idea, and I had no idea what I wanted. He just knows what I like.''

She said her fiance ''worked with the girls that I have worked with for years and years and years, so he kind of was picking the specialists' brains. He kept it within a very small circle of people. It was one of the sneakiest jobs I've ever seen, actually.''

Jennifer became emotional when talking about her fiance during her first post-engagement interview with close pal Chelsea Handler last week and couldn't stop herself getting teary as Chelsea gushed about what a great couple they are.

The surprise proposal - which took place just over a year after they started dating - occurred when the couple were dining at Greenwich Village restaurant Blue Hill in New York.

After the proposal, an ''overwhelmed'' Jennifer - who was previously married to Brad Pitt - called her close friends and family to share the good news.

A source said: ''Just after Jen got engaged, she called all her friends and told them about it. She was so happy and said she loves the ring. I think she was overwhelmed and overjoyed by the size and beauty of it!''