Ineos is not bluffing with its plan to shut Scotland's Grangemouth oil refinery within weeks unless workers agree by a Monday deadline to accept proposed cuts in terms and conditions, the chemical company's chairman said.

"It's really simple. The employees have a decision to make," Jim Ratcliffe told the Sunday Times newspaper. "They either want a future for Grangemouth and are prepared to accept change or they wish to continue to inflict damage on the company."

"This is not a bluff. The clock is ticking," Ratcliffe said.

Meanwhile the Unite union organized a protest outside the plant on Sunday, involving hundreds of workers, against the proposed changes, which include cuts in pension benefits.

It also took out full-page adverts in Scottish newspapers saying Ineos was holding the country to ransom.

Politicians of major parties have urged both sides to get back around the negotiating table to avert the possible permanent closure of the refinery and petrochemical plant that together employ well over 1,000 people.

Swiss-based Ineos said on Wednesday it had closed the 210,000 barrel per day refinery and petrochemicals complex pending an outcome to the dispute.

The future of the refinery is being closely watched as it supplies most of the fuel for Scotland and provides steam and power to BP's Kinneil oil terminal, which processes North Sea crude that comes ashore via the Forties Pipeline System.

A permanent shutdown could herald a new wave of closures in the sector as the industry battles rising competition from new plants in Asia and the Middle East and dwindling domestic demand.

The union has said it cannot talk until the ultimatum on pensions has been withdrawn, and the company said it will not talk until all threat of strike action has been removed.

Unite's Scottish secretary Pat Rafferty said in a comment in the the Herald Scotland that the union had offered to suspend industrial action so talks could convene with the plant working.

"The company walked from these talks only to issue "sign or be sacked" letters to staff. Clearly, the threat to impose seriously detrimental new terms of employment - the first step to wholescale casualisation at Grangemouth - on the workforce must be withdrawn for any meaningful progress on the future of the plant to ensue."

A union source said the deadline for acceptance of the company's planned changes was 6.00 p.m. local time on Monday.

He said the union, which disputes the company's analysis of the plant's financial situation, was due to meet Ineos representatives in the morning but he did not expect much to come from it.

Ratcliffe told the Sunday Times: "This (shutdown of the plant) has already cost a minimum of £20 million ($32 million). If it stays shut, Grangemouth runs out of money quite quickly on that basis. Ineos won't continue funding beyond a reasonable amount of time and certainly has no obligation to."