Europe\'s largest independent oil refiner Petroplus, seeking to restore $1 billion in frozen credit lines, will next week shut its oil refinery near Rouen in France, a union official said Friday. But the process of shutting down the Swiss firm\'s refinery at Petit-Couronne will \"be long and could last from Monday to Friday,\" Yvon Scornet from France\'s leading CGT trade union said. Some fear the company may close one of its plants as it struggles to secure funds to maintain its operations. Scornet said other refineries facing the axe are Coryton in Britain, Antwerp in Belgium, Ingolstadt in Germany and Cressier in Switzerland. Petroplus announced on Tuesday that its lenders had frozen $1 billion (765,000 euros) in uncommitted credit lines -- funds it said were \"critical\" for it to continue operations. The news sent shares on the Swiss market tumbling and on Thursday credit ratings agencies Standard and Poor\'s and Moody\'s slashed the firm\'s rating, indicating it was vulnerable to default. Petroplus Holdings AG is headquartered in the city of Zug near Zurich and headed by Frenchman Jean-Paul Vettier. The group, which employs 2,500 people, indicated at the beginning of December that it faced increasing liquidity problems. Its five refineries, which include the Antwerp plant in Belgium, have a combined capacity of about 667,000 barrels a day.