Business Secretary Greg Clark has been criticised for waiting until the "last minute" to seek assurances from Melrose for its proposed takeover of GKN.
He has written to Melrose demanding "binding" commitments for its £8.1bn bid for the UK engineering giant.
Shadow business secretary Rebecca Long-Bailey said Melrose's response to his letter offered "very little certainty and raised even more questions".
She also questioned whether its assurances were "legally enforceable".
Mr Clark has sought "extensive and clear" measures over GKN's workforce, research and development, and pension schemes.
In response, Melrose set out a number of pledges, including not selling GKN's aerospace division for five years.
The turnaround specialist said that for five years it would maintain the company's London listing, maintain its UK headquarters, and ensure research and development (R&D) spending was at least 2.2% of sales.
Labour MP Rachel Reeves, who chairs the Business select committee, also questioned how long Mr Clark had taken to intervene in the proposed takeover.
At a hearing on Tuesday, she asked the minister: "Why did it take until three days before shareholders have to vote on this bid for you to write to Melrose to get some assurances, which are frankly pretty limited and it's too late in the day now for you to try and drive a harder bargain?"
"The secretary of state says that it is still possible to call this in, but this takeover has been hanging over GKN and their employees and their wider stakeholders for more than two and a half months now."
In a statement to the stock market, GKN boss Anne Stevens said: "Melrose is asking shareholders to consider a set of last-minute undertakings which leave considerable uncertainty and do not adequately address the need for long-term shareholder value creation.
"The Melrose offer remains the low-value, high-risk option for shareholders."
GKN is one of the UK's largest industrial firms, employing more than 59,000 people globally - 6,000 of them in the UK.
It makes parts for planemakers Airbus and Boeing, as well as parts for Volkswagen and Ford cars. It also holds major defence contracts, including making parts for the F-35 stealth fighter
Mr Clark told MPs that he would await Ministry of Defence advice before deciding whether to intervene in the deal on national security grounds.
If Melrose's bid succeeded, he said the MoD would seek a "legally binding commitment relating to the management of any defence contracts".
GKN shareholders have until midday on Thursday to accept the bid from Melrose, or back GKN's own business plan.
The plan includes combining GKN's automotive unit with US group Dana, leaving GKN to concentrate on its aerospace business.
Melrose specialises in buying up industrial companies it believes are undervalued and restructuring them before selling them on.
In his letter to Melrose boss Simon Peckham, Mr Clark set out a series of commitments "which would need to be binding" if the company's bid for GKN was successful.
In response, Mr Peckham said Melrose had agreed a number of legally enforceable undertakings with the Takeover Panel to address the government's concerns - including maintaining GKN's UK base and its R&D spending.
"To demonstrate the strength of our commitment and ensure that its improvement and investment programme is not unduly interrupted, we are willing to make a legally binding commitment to you ... that, subject to below, Melrose will not sell the aerospace division before 1 April 2023."
Analysis: Simon Jack, BBC business editor
With two days left in which shareholders must decide whether to accept the bid - how many who liked the look of the Melrose bid will think again now that the company has tied its hands more than it may have liked?
Not many, according to one big shareholder who spoke to the BBC. Certainly, the government thinks that Melrose has a good chance - and wanted to makes its concerns known while it still can.
Whatever happens between now and Thursday, the future of GKN will look very different.
Investors have until Thursday lunchtime to decide - 50.01% is the level of support needed.