The Catholic Church in Sydney is sending letters to companies that publicly support same-sex marriage, expressing its "grave concern" about their stance.

The Archdiocese of Sydney has targeted some of the 150 businesses that put their names and logos to recent newspaper advertisements supporting gay marriage.

The ABC has seen a copy of one letter addressed to the chairman of partners at Maurice Blackburn law firm, Steve Walsh, from the Archdiocese's business manager Michael Digges.

"It is... with grave concern that I write to you about the Marriage Equality for Australians campaign," it reads.

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"You are publically supporting a strategic, political and well-funded campaign designed to pressure the Federal Government into changing the Marriage Act.

"For corporations to speak on such issues... is indeed overstepping their purpose and it is to be strongly resisted."

Maurice Blackburn principal Liberty Sanger said she was not sure what the intention of the letter was, but either way the law firm would not be intimidated.

"Now it may well be that their intention was to try and frighten us into not participating in the debate," she said.

"If that was the objective, well it's had... obviously had the opposite effect.

"I think the important thing is that we continue to show our support so that others who have the same view as us have the courage to speak up and encourage parliamentarians to make the right decision in the Parliament."

Liberty Sanger defended the advertisements supporting the marriage equality campaign.

"I thought it was a very heavy handed response and uncalled for, because we're very respectful of everybody's points of view in this debate," she said.

Church a 'significant user of goods and services'

In the letter, Mr Digges also reminds Maurice Blackburn of the church's purchasing power.

"You may be aware that the Catholic Archdiocese of Sydney is a significant user of goods and services from many corporations, both local and international," it reads.

Ms Sanger said the comments were not helpful.

"If it was his intention to make us think that he was going to be able to keep Catholics away from Maurice Blackburn because of our support for marriage equality, then I think he will be sadly mistaken," he said.

"Whether or not it was intended as a threat, I'll leave for him to comment upon."

Ms Sanger said the letter would not change the company's views on same-sex marriage.

The Football Federation of Australia has confirmed it received the same letter.

A spokesperson for the organisation said it does not change its stance on marriage equality.

The ABC requested an interview with Michael Digges but he was "unavailable".