Grangemouth's giant refinery and petrochemical complex was being fired back up last night amid relief across Scotland that owners Ineos had decided against closure and ended their week-long lockout of workers.

But even as politicians congratulated each other on the outcome the mood was charged with anger from the Scottish Trade Union Congress at the company's actions and bitterness at the sacrifices forced on the workers.

Welcoming news that billionaire Jim Ratcliffe - the major shareholder in Ineos - would reverse Wednesday's blitzkrieg decision to cut 800 jobs and close the petrochemical site, Unite Scottish secretary Pat Rafferty said: "Relief will ring right round the Grangemouth community and across Scotland.

"Hundreds of jobs that would have been lost can now be saved and £300 million will be invested into the plant."

Mr Ratcliffe's reversal of his decision to axe the plant followed Thursday's dramatic acceptance by Unite of a "survival plan" for the business, including a three-year pay freeze, ending of the final-salary pension scheme and a no strike deal.

Mr Rafferty said: "Today's news is tinged with sadness - decent men and women are being asked to make sacrifices to hold onto their jobs - but the clear wish of our members is that we work with the company to implement its proposals."

Scottish politicians back-slapped as the Grangemouth workers were being forced into massive concessions to persuade rapacious owner Mr Ratcliffe to reopen the plant.

First Minister Alex Salmond said the news was "a tremendous fillip" for the workforce.

Mr Salmond said it was "a day of great satisfaction that not only has a key part of Scotland's industrial infrastructure been saved but that people can look forward with confidence to a bright future."

Scottish Secretary Alistair Carmichael agreed: "This is the outcome we have worked towards and one which Scotland and Grangemouth deserve."

But Scottish TUC general secretary Grahame Smith sounded an angry note as he accused Ineos of effectively going on strike by closing the plant down.

"The approach of Ineos has flouted all of the norms of collective bargaining and the company effectively went on strike against its own workers, the local community and the people of Scotland," he said.

Mr Smith called on the Scottish and Westminster governments to co-operate to protect the future of Grangemouth.

"A plant of such critical importance to the local and Scottish economy requires the active support of elected governments and the fullest possible involvement of its workers and the wider community."

The Scottish government is to support Ineos's application for a £9m grant and the Westminster government has given "pre-qualification approval" for a £125m loan guarantee.

Ineos Grangemouth Calum MacLean chairman refused to rule out job losses despite the pledge of millions of pounds in public support, although he insisted any redundancies would be "very limited."