Heidi Montag 'threatens to sue Pratt'

Heidi Montag has reportedly instructed her lawyers to pursue legal action against her estranged husband Spencer Pratt.

Montag told TMZ that she is upset that Pratt plans to write a tell-all book about the couple's marriage and will attempt to block its publication.

''This is exactly why I left him... Right now I'm looking into my legal options," said the reality TV star.

It was previously reported that Pratt's book will include alleged details of Montag's difficult relationship with her mother, Darlene Egelhoff.

The couple's divorce will be finalised in February 2011.