The 'G.I. Joe' actress wants her Welsh lover to bulk up his frail frame before they get married this summer.
A source said: "Rhys has always been quite skinny. Sienna loves him but she just wants him to tone up a bit so he looks great for their big day."
Sienna, who has previously dated Jude Law and Daniel Craig, has bought a £3,500 Power Plate exercise machine for Rhys so he can work out at home.
The Power Plate, which is also used by Madonna and Amy Winehouse, uses vibrations to tone up muscles and improve fitness.
The source added to Britain's Daily Mirror newspaper: "Rhys is already using the machine. He wants to make Sienna happy. She is in Los Angeles and has left him instructions to exercise. She thinks they should both be keeping fit."
Sienna, 26, finally agreed to be Rhys' wife in February after the 39-year-old 'Notting Hill' actor popped the question several times.
The couple are believed to be planning to marry on a private Mexican island.
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Sienna Miller
Rhys Ifans
