The Prime Minister told senior ministers at Cabinet that it was important for the Government to resist amendments to the Data Protection Act which could "undermine our free press".
After spending almost £50 million of taxpayers' money on investigating phone-hacking, it would not be "proportionate" to launch further inquiries, Mrs May said.
And she said it would be "unnecessary and disproportionate" to require publishers to sign up to an approved regulator or face potentially massive costs in court cases, even if they win.
Announcing the Leveson Inquiry into the ethics and practices of the press following the phone-hacking scandal in 2011, then prime minister David Cameron said that a second phase of the inquiry would be launched once police investigations and court cases were concluded.
In response, former Labour leader Ed Miliband tabled an amendment to the Data Protection Bill to establish a new statutory inquiry into the media.
And a second amendment, tabled by Labour's deputy leader Tom Watson, would see publishers not signed up to a state-supported regulator pay their own and their opponent's legal costs in data protection cases, even if they win.
Mrs May's official spokesman told reporters: "We have set out the importance of these votes and of the Government resisting those amendments and we would hope that when MPs come to consider these amendments, they will look at the strength of the case that we've put forward."
The spokesman said that Mrs May told Cabinet that it was "very important for the Government to resist amendments which could undermine our free press".
After almost £50 million of public money has been spent on investigating phone hacking, the PM told ministers that establishing a further public inquiry, requiring great time and expense, would not be a "proportionate" solution to allegations that have already been the subject of several extensive police investigations or ongoing investigations by the Information Commissioner's Office, said the spokesman.
The Government remains committed to a voluntary system of press self-regulation, Mrs May told Cabinet. It was "unnecessary and disproportionate" to require the press to sign up to a system which has already been outright rejected by the majority of publications, she said.
This was particularly the case, when an independent and strengthened system of regulation is now in place, with Ipso making continued improvements, such as the introduction of a mandatory arbitration scheme in line with Lord Justice Leveson's recommendations, the Prime Minister told Cabinet.
Many people would consider it "against natural justice" that, even if a newspaper was found not to be at fault in a court case, they could still end up having to pay costs, she said.
Local newspaper editors have warned fresh attempts to tighten press regulation would cause "irreparable damage" to the industry.