PTT had revealed Wednesday that it had struck a deal to buy Cove Energy for £1.22 billion ($1.92 billion, 1.50 billion euros) after increasing its bid to 240 pence per share, from an earlier bid in February of 220 pence.
The announcement was a blow to Shell, which last month that said its Bidco subsidiary had reached an agreement to buy Cove Energy for £1.12 billion, or 220 pence per share.
"Shell Bidco notes the recent announcement made by PTT Exploration and Production Public Company Limited of its offer for Cove," it said in a statement.
"Shell is currently considering its options and will make a further announcement if appropriate."
Shell also extended its offer deadline until June 13, after receiving valid acceptances representing just 4.83 percent of the existing share capital of Cove.