Around 1,000 jobs at the Coryton oil refinery in Essex are under threat after Swiss owner Petroplus said it would file for bankruptcy.
The refinery, which supplies 20% of fuel for south-east England, halted sales on Monday and told staff it was not sure when supplies would restart.
East of England MEP Richard Howitt said he feared petrol supplies would be affected.
The union Unite said the owners and government needed to take joint action.
Petroplus has said it will file for insolvency after failing to reach an agreement with creditors to extend deadlines for loan repayments.
The firm said it would make the filing "as soon as possible".
There are around 500 staff and 350 contractors employed at the Coryton refinery.
As well as refining oil for use as fuel, it also imports fuel from other countries which has already been refined.
East of England MEP Richard Howitt says that he feels "hugely sorry" for the Coryton workforce
Linda McCulloch, from Unite, said: "One thousand jobs are at risk but we firmly believe that joint action by the owners and government can help secure the business."
Her colleague, Unite regional officer Russ Ball, said: "Crude oil is still being produced and the refinery is operating, but nothing is going out of the gate."
The site produces petrol, diesel and other clean fuels.
Mr Howitt told BBC Radio 5 Live: "One thousand job losses in Essex will have a devastating impact on the local economy.
"I don't want to be alarmist about this, but I don't want to be dishonest either. Supplies across London and the South East could be affected and I have been told this could impact the Olympics."
A Department of Energy and Climate Change spokesman said: "The refinery remains operational.
"We understand that a process is under way to put in place the necessary commercial arrangements to deliver product into the market.
Companies have already made alternative arrangements to ensure adequate supply of products are available while these commercial arrangements are being put in place."
The UK subsidiary of the firm, which owns the Coryton oil refinery in Essex, has appointed PricewaterhouseCoopers as administrators.
Steven Pearson, on behalf of the administrators, said: "Our immediate priority is to continue to operate the Coryton refinery and the Teesside storage business without disruption while the financial position is clarified and restructuring options are explored."
He said there were plans to have a number of discussions during the next few days over the future of the site in Coryton and the business in Teesside.
BP is a major customer of the refinery in Coryton and said it would be watching the situation very closely.
ExxonMobil, the owner of the Fawley Refinery on Southampton Water, told the BBC its stock levels for London and the South East remained good and it would continue to deliver to forecourts as normal.