Scottish Labour leader Johann Lamont made the call last night after talks with Unite and Ineos about the dispute at Grangemouth, which threatens fuel supplies in Scotland.
Her comments came after First Minister Alex Salmond had warned the future of the refinery was at risk unless the two groups reached a "truce deal" that would enable it to start running again.
Ms Lamont said: "I have spoken to both the union and the company and as a result of the union undertaking that there will be no strike action up to the end of the year the company should now withdraw the ultimatums they have delivered to the workforce.
"This is not the way we should be engaging in workplace disputes in the 21st century.
"The company should also undertake to immediately reopen the plant and return to production and both the union and the company should return to meaningful talks, with Acas if necessary, to deliver a solution to this matter which has serious consequences for Scotland and the UK."
Earlier, Mr Salmond reiterated a statement he made late on Thursday night following his own talks with both parties. He called on Unite to commit, in writing, to normal working at the plant without industrial action until the end of the year and called on Ineos to fire up the plant.
The longer the site is "idle and cold", the more difficult things will become, Mr Salmond said.
The dispute which resulted in a vote for strikes, which were called off, centred on the treatment of union official Stephen Deans but the future of the plant now appears to be at stake.
Mr Salmond added: "If we can get the plant started and working again that would be a better atmosphere perhaps to have another try at completing this final mile of negotiations.
"However, the alternative is also the case. If the plant stays down and we stay in the war of words, the exchange of press releases, interviews and tweets, then there is a risk that the situation will deteriorate further. And the fundamental risk is the future of the Grangemouth plant."
Ineos said it would restart production if Unite gives an assurance there will be no industrial action while the company consults workers over a so-called survival plan.
The union is said to have made such an offer at an Acas meeting earlier this week, but Mr Salmond said it should put the commitment in writing.
Unite is taking out full page newspaper advertisements in the coming days in an attempt to win public support for its stance.
In what it says is a "message to the people of Scotland' they will claim that Ineos owner Jim Ratcliffe should bear full responsibility for the dispute which its adds, risks destroying "good Scottish jobs.
Mr Salmond said: "Unite were good enough to share with me the points that were put forward within Acas.
"There was also other issues which unfortunately broke the talks down, but it does seem to me that this was a point that might be very important because the union's position is they are not going to go on strike, they are not in dispute at the moment.
"The company's position is that they can't refire the plant because (the union's) position might change, next week or the week after or whenever they can reinvoke industrial action, and therefore it wouldn't be safe to boot up the plant."
He suggested Unite comply with the company's request for the commitment in writing.