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The former model parted company with Mischon de Reya last November and it later emerged that she could no longer afford to pay for its services.
The London-based firm, which represented Mills for 18 months, has reportedly threatened to take legal action against her over alleged unpaid legal bills of £2 million.
However, a souce now claims that Mills has responded with legal action of her own, saying: "There is a possibility of Heather Mills counter-suing. Whatever she owes Mischon - whether it's £1m or £2m - one would have expected her to get something for it. There has been no divorce settlement and no outcome over all this time.''
Mills is back in court next week as part of her ongoing divorce battle with Sir Paul McCartney. She is expected to represent herself and cross-examine her husband during the hearing.
