A businessman is suing his lawyers after his former de facto partner was awarded his Porsche, a cemetery plot and $900,000 following their separation.

The Adelaide man alleges that South Australian-based law firm Grope Hamilton are liable for the losses he claimed he suffered after they are alleged to have failed in their duty of care, according to media reports.

In July 2003, the 64-year-old businessman had a cohabitation agreement drawn up by the law firm just one year after he started a de facto relationship.

A businessman is suing his lawyers after his former de facto partner was awarded his Porsche, a cemetery plot and $900,000 following their separation

The Adelaide man was ordered to give his Porsche 911 to his former de facto partner after their split

According to legal documents lodged with Supreme Court, he alleges the pair's assets agreement was officially witnessed by their cleaning lady instead of a Justice of Peace or a solicitor.

However, the couple, who ended their relationship in early 2010, had their agreement rejected by a court in September 2011 after it was ruled to be invalid.

The court ordered him to give his former partner a Porsche 911, a cemetery plot and $900,000.

The plaintiff alleges that Grope Hamilton owed him 'a duty of care to perform the legal services with the skill care and diligence as is reasonable to expect from a law firm of legal practitioners professing to have the expertise, skills and qualifications of Grope Hamilton.'

He had an agreement drawn up by the law firm just one year after he started a de factor relationship

According to legal documents lodged with Supreme Court, the businessman revealed the pair's assets agreement was officially witnessed by their cleaning lady

He is also suing his former partner's law firm de Groots claiming they failed to ensure the agreement was valid

However, in its defence the firm alleged the man was aware of the requirements but had refused to attend meetings to discuss the agreement and failed to have it witnessed by a JP or a solicitor.

According to media reports, he is also suing his former partner's law firm de Groots claiming they failed to ensure the agreement was valid.

Grope Hamilton states both the businessman and de Groots knew Grope Hamilton did not practice in family law and the onus was therefore on de Groots to provide advice to him in connection with the agreement.

De Groots have filed a defence claiming that the man did not seek any advice from them, and stating they owed no duty of care to him.

The matter will return to court at a later date.