"Importantly, if we do not respond legislatively within the year, most adult prostitution-related activities will be decriminalized," MacKay said in his opening statement at the House justice committee hearings on Bill C-36.
MacKay is the first witness, kicking off this week's marathon round of hearings on the Harper government's new prostitution bill.
The Supreme Court of Canada struck down Canada's old prostitution law last December and gave it a year to replace it with one that would comply with the Charter of Rights and Freedoms.
MacKay says he's open to amending the bill, but he's dropping strong hints that will have limits.
He says the bill is constitutionally sound and is an adequate response to the Supreme Court.
MacKay told the committee that the government doesn't accept the idea that prostitution is inevitable and that prostitution victimizes the communities in which it takes place, as well as the women who sell sexual services.
"The government maintains that prostitution's inherent harms and dangers would only grow and be exacerbated in a regime that perpetrates and condones the exploitation of vulnerable individuals through legalized prostitution," he said.
"Bill C-36 reflects a fundamental shift towards the treatment of prostitution as a form of sexual exploitation that disproportionately and negatively impacts on marginalized groups and individuals."
60 witnesses over 20 hours of hearings
Justice Department officials, who advised the government, will be open to questioning by all parties after MacKay has finished his testimony later on this morning.
Prostitution itself was actually legal in Canada under the old law, but most related activities — including communicating in a public place for the purposes of prostitution, pimping and running a brothel — were criminal offences.
The Supreme Court said that amounted to a violation of the basic Charter right to security of the person was concerned that the provisions unduly increased the risk to sex workers.
The Conservatives' new bill creates new offences for clients and pimps, but does not criminalize prostitutes themselves.
It also cracks down on advertising and selling sexual services in public places where a child could reasonably be expected to be present.
Parliamentary committees rarely convene during the summer recess.
This week, the committee expected to hear from more than 60 witnesses over 20 hours of hearings set to begin today and run until Thursday morning.
The vast list of those testifying includes sex workers, indigenous women, community workers and experts from Europe.