Joy Williams, 69, lived with Norman Martin for almost two decades before his death, but he remained married to Maureen Martin.
Mr Martin, a former dentist, died of a heart attack in 2012.
Lawyers for Ms Williams said the case highlighted the need to update the laws on co-habitation.
The firm Irwin Mitchell said that their client "risks losing her home".
Mr Martin and his partner Ms Williams owned a three-bedroom home in Dorset as tenants in common - which means his share of the house was not automatically transferred to his partner.
Mr Martin did not divorce or update his will to "reflect his relationship with Ms Williams", her lawyers said.
Both Mrs Martin and Ms Williams attended the start of proceedings at Central London County Court, where a judge is being asked to make a ruling on the dispute.
The case is expected to continue until Thursday.
A woman who lived with her partner for 18 years has launched a legal battle after his share of their home passed to his estranged wife when he died.