Kiera as Diana?

According to one source, a film biography of Princess Diana will be released next year to coincide with what would have been her 50th birthday. The film supposedly will star Kiera Knightley as the Princess of Wales and Helen Mirren has her mother (Shouldn't she be reprising her role as Diana's mother-in-law instead?). The movie is going to cover Diana's early life, her marriage to Charles, their separation and her death in 1997 in Paris.