Theresa May is urging MPs to reject a bid to further tighten press regulation ahead of a knife-edge vote on new proposals in the Commons.

The prime minister told senior ministers at cabinet that plans put forward by opposition MPs would “undermine” the free press and unjustly see papers forking out crippling sums of money even if they do not lose legal cases.

But government insiders have suggested there is a serious chance of the proposals passing, with questions over how the DUP may vote and some Tories planning on abstaining.

It comes after local newspapers warned many of their number will be forced to close if the plans are passed.

Ms May’s spokesman said: “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.”

When David Cameron announced the Leveson Inquiry into the ethics and practices of the press following the phone-hacking scandal in 2011, he said a second phase would be launched once police investigations and court cases were concluded.

But there was fury from some celebrities and privacy campaigners when Culture Secretary Matt Hancock announced in March this year that the second phase was being ditched.

Government announces it will not be pursuing a Leveson 2 inquiry

In response, former Labour leader Ed Miliband tabled an amendment to the Data Protection Bill to establish a new statutory inquiry into the media.

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.

Ms May’s spokesman said: “Almost £50m of public money has already been spent on investigating phone-hacking, and establishing a further public inquiry requiring great time and expense is not a proportionate solution to allegations which have already been the subject of several extensive police investigations or ongoing investigations by the Information Commissioner’s Office.

“The prime minister said [at cabinet] the government remains committed to a voluntary system of independent press self-regulation.”

He added: “The prime minister said many would consider it against natural justice that, even if a newspaper was found not to be at fault, it could still end up having to pay costs.”

Explosive evidence on first day of Leveson Show all 4 1 /4 Explosive evidence on first day of Leveson Explosive evidence on first day of Leveson Pg-2-leveson-2-.jpg General views of News International building Pennington Street, Wapping London Explosive evidence on first day of Leveson Pg-2-levesonbrooks afp-jpg.jpg In 2009, Rebekah Brooks ended her editorship of The Sun to become News International chief executive AFP Explosive evidence on first day of Leveson Pg-2-leveson4-rex.jpg Many of the new leads come from the notebooks of jailed investigator Glenn Mulcaire Rex Features Explosive evidence on first day of Leveson Pg-2-leveson1-ap.jpg Colin Myler was editor of the News of the World AP

Local newspaper editors have warned fresh attempts to tighten press regulation would cause “irreparable damage” to the industry.

In a survey of carried out by the News Media Association, 92 per cent of local editors said they did not think another “Leveson-style” inquiry into the media should take place, with the remaining 8 per cent saying they were unsure.