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Ben Affleck's mother has become the latest person to let slip about her daughter-in-law Jennifer Garner's baby news, reportedly confirming Hollywood's worst-kept secret at a family wedding.

When her actor son and his wife failed to make it to the Cape Cod nuptials on July 12, Chris Affleck told nosy guests that Garner "wasn't up for the trip".

And when guests pressed her for more information, grandma Chris left them with no doubt there is soon to be another family addition.

An eyewitness tells Life & Style magazine, "She put her hands on her tummy, patted it gently and gave a huge smile. Then she leaned in and said 'Baby number two!'"

Days later, the couple's close friend, actor Victor Garber, also let the baby news slip, but he has since insisted his comments were taken out of context and he has no idea if Garner, his former Alias co-star, is pregnant.