Tanker drivers returned to work on Tuesday amid renewed efforts to stave off another possible four-day strike over pay.
The drivers, who work for two haulage firms contracted to Shell, walked out last Friday, leading to hundreds of garage forecourts running out of diesel and unleaded petrol.
They have threatened another four-day strike on Friday if the talks fail.
"I think there will be a conclusion today," said a spokesman for the union Unite. "I don't know whether it will come to a successful conclusion."
The drivers say their average basic salary of 32,000 pounds has not gone up since 1992.
"It was 32,000 pounds on their payslips in 1992 and it is 32,000 pounds now," the union spokesman said.
The haulage companies, Hoyer UK and Suckling Transport, said unions had rejected a pay offer which would have taken the drivers' average salary to 41,500 pounds, media reports said.
Unite was reported as saying that excluding overtime, the offer would have increased salaries to 36,000 pounds.
The union spokesman on Tuesday refused to say what the drivers were demanding, saying only that it was a significant advance.
Another four-day strike could affect hundreds of garages.
More than 600 of the UK's 8,700 petrol stations had run out of unleaded and diesel fuels on Monday, with the south-west of England, Cardiff and parts of London and the West Midlands particularly affected, the BBC said.
One Devon petrol station had begun charging 1.99 pounds a litre - more than 9 pounds a gallon - for unleaded and diesel, it added.
