British Foreign Secretary Philip Hammond said Thursday on a visit to the Philippines that any attempt to restrict air and sea travel in the disputed South China Sea would be viewed as a "red line".
China said it landed three planes over Fiery Cross Reef in recent days, prompting protests from rival claimants Vietnam and the Philippines, and raising fears it could impose military controls in the area.
"Freedom of navigation and overflight are non-negotiable. They are red lines for us," Hammond told a joint news conference with his Filipino counterpart, Albert del Rosario, in Manila.
Hammond, whose Manila visit followed a trip to China, did not elaborate on what action would be taken if the "red lines" were crossed, other than to say Britain would continue to assert its right to sail in the area.
Del Rosario said he was worried that, with the test flights, China was laying the groundwork for the declaration of an Air Defence Identification Zone (ADIZ), similar to the one it declared in the East China Sea that riled Japan.
"If this is not challenged, China will take the position that the ADIZ could be imposed and whether this is done in terms of a de facto basis or whether it is official, this will be deemed as unacceptable to us," Del Rosario said.
Vietnam, another claimant in the South China Sea, has also condemned the test flights as a violation of its sovereignty.
China has alarmed its rivals with its massive reclamation and construction of facilities on disputed reefs, including a 3,000-metre (9,842-foot) runway on Fiery Cross in the Spratly island group, which is claimed by Vietnam.
Chinese foreign ministry spokeswoman Hua Chunying claimed Saturday after announcing the first test flight that it was "civilian" in nature.
The Philippines has asked a United Nations-backed tribunal to void China's claim over almost the entire South China Sea. It expects a decision this year.
China did not participate in the arbitration hearings at The Hague as it maintained that sea disputes should be resolved bilaterally.
"Win or lose, we will abide by the rule of law and we expect China to do the same," Del Rosario said.
Hammond said Britain would not take sides on the dispute but appealed to claimants to resolve their differences under international law.
"We recognise the tribunal and we will recognise the decision of the tribunal," Hammond said.
Malaysia, Brunei and Taiwan also have competing claims in the South China Sea, which hosts vital shipping lanes over vast oil and gas reserves.