Washington has said it is “deeply troubled” by a controversial media bill in Poland that with its chaotic passage through parliament has raised questions over the ruling Law and Justice party’s (PiS) long-term prospects.

The US secretary of state, Antony Blinken, said the bill, widely seen as targeting Poland’s largest independent TV station, TVN, whose news channel TVN24 has often been critical of the populist government, “threatens media freedoms”.

Blinken said in a statement that a free and independent media was fundamental to the bilateral relationship between the two countries, warning that commercial investment could also be undermined by “Poland’s troubling legislation”.

The Polish prime minister, Mateusz Morawiecki, on Thursday denied the government was targeting a specific channel, telling a news conference the move was “just about tightening the regulations” on foreign media ownership.

The bill, if it becomes law, would prohibit non-EU companies from holding a controlling stake in Polish media and could force the US media group Discovery to sell its majority holding in TVN, whose licence expires at the end of next month.

The bill was “an unprecedented attack on freedom of speech and the independence of the media”, the company said, adding that Poland’s “future as a democratic country in the international arena and its credibility in the eyes of investors” were at stake.

Polish business leaders also warned of the bill’s possible impact on transatlantic relations. “The economic and social consequences will be very negative,” said Maciej Witucki of the Lewiatan business confederation.

“It is very likely that before any American company invests even one dollar in Poland, it will think 10 times. It is also a warning signal for other potential foreign investors from other countries that investments in our country are burdened with high risk.”

Protesters gathered outside parliament in Warsaw on Wednesday night after Polish MPs voted by 228 votes to 216 in the 460-seat lower house after a stormy session in which the bill’s passage was briefly held up by a furious opposition.

It came after Morawiecki on Tuesday fired his deputy, Jaroslaw Gowin, the leader of the junior coalition member Accord, prompting the party to leave government and depriving the PiS-led coalition of its majority.

However, PiS persuaded enough MPs from smaller parties – including Accord – to defect. Polish media reported cash and other inducements were on offer, with Gowin saying his MPs had been offered government posts and funds for their constituencies.

But the party’s fragility was confirmed as it lost several key votes during the session, including one to postpone a vote on the media law. The speaker, Elzbieta Witek, a PiS member, ordered another vote, which the government won, drawing cries of “treason” and “fraud”, and prompting the opposition to declare the vote illegal.

The bill now goes to the senate, which must vote on it within a month and where it is expected to be defeated. “The democratic majority in the senate will never approve an attack on independent media,” tweeted Tomasz Grodzki, the senate speaker. “We will uphold freedom of speech.”

It would then return to the lower house, or Sejm, where the government would need an absolute majority to pass it. Analysts were divided on Thursday as to whether a wounded PiS would be able to summon the votes, with the far-right Confederation party already saying its 11 MPs would abstain.

Blinken also called on Poland not to proceed with another controversial piece of legislation that is likely to make it harder for Jews to recover property seized by Nazi German occupiers during the Holocaust and kept by postwar communist rulers.

Morawiecki said the law was implementing a 2015 ruling by Poland’s constitutional tribunal that a deadline must be set after which faulty administrative decisions can no longer be challenged.