More than 50 of Australia’s biggest businesses have joined the chorus calling for marriage equality in a full-page newspaper ad.

Among the 53 companies listed in the ad, which appeared in today’s edition of The Australian, are giants Google, Qantas, Optus, Telstra and David Jones.

The advertisement was produced by activist group Australian Marriage Equality and called for more corporate support on the issue.

On their website, the group published an open letter from business leaders outlining the reasons behind their backing of same-sex marriage.

“The rights and liberties of all individuals are sacrosanct,” the letter reads.

“They are essential in the creation of a healthy, harmonious and open society.

“Equality in the workplace works; Discrimination does not.

“We support the right for all our employees to have equal opportunities in life. We therefore support marriage equality.”

Bill Shorten and Tanya Plibersek have formally called for an amendment to the Australian Marriage Act. (9NEWS)

Opposition Leader Bill Shorten has indicated he will introduce a private members bill on Monday to parliament to call for the Marriage Act to be amended to allow same-sex couples to wed.

Do you think Australia's marriage laws should be changed to allow same-sex marriage? Yes 33658017 No 1809995