Spencer Pratt and Heidi Montag. (WENN.COM file photo)

The Hills star Heidi Montag has filed for divorce from her estranged husband Spencer Pratt.

Montag walked out on her former reality TV co-star in May and filed for legal separation in June.

She has now moved to make the marriage split official after lodging a divorce petition in Santa Monica, California Superior Court on Friday, citing irreconcilable differences.

Montag's lawyer, Jodeane Farrell, confirms to People.com: "Heidi has amended her petition for separation and today has filed a petition for dissolution of marriage from Spencer Pratt.

"The couple has agreed they would like their divorce to be finalized in a timely manner in an out-of-court settlement. Both parties are amicable with each other and over the possibility of finalizing their divorce."

Montag and Pratt wed in Mexico in 2008 and exchanged vows for a second time on TV in 2009 amid claims their initial nuptials were not legal and were part of an elaborate publicity stunt.