A Royal Navy warship has sailed close to islands claimed by Beijing in the South China Sea, a move denounced by China as a "provocation".
In a sign of Britain increasingly flexing its military muscle in the region, HMS Albion last week passed by the Paracel Islands, where it was reportedly confronted by the Chinese military.
The Albion, a 22,000 ton amphibious warship carrying a contingent of Royal Marines, was on its way to Ho Chi Minh City, where it docked on Monday after a deployment in and around Japan.
Beijing dispatched a frigate and two helicopters to challenge the British vessel, but both sides remained calm during the encounter, a source told Reuters.
China said Britain was engaged in "provocation" and that it had lodged a strong complaint. In a statement to Reuters, the Foreign Ministry said the ship had entered Chinese territorial waters around the Paracel Islands on August 31 without permission, and the Chinese navy had warned them to leave.
Q&A | South China Sea dispute
"The relevant actions by the British ship violated Chinese law and relevant international law, and infringed on China’s sovereignty. China strongly opposes this and has lodged stern representations with the British side to express strong dissatisfaction," the ministry added.
"China strongly urges the British side to immediately stop such provocative actions, to avoid harming the broader picture of bilateral relations and regional peace and stability," it said.
"China will continue to take all necessary measures to defend its sovereignty and security."
Nick Childs of the International Institute for Strategic Studies said the Chinese Navy was becoming more assertive and regularly shadowed vessels in the South China Sea.
"No side has much to gain by deliberately sparking a clash, but the balance of power has definitely shifted towards China," he said.
"The maritime domain has become increasingly contested globally," he says and suggests the incremental development and militarisation of islands in the region by China will only increase tension.
A Royal Navy Spokesperson said: “HMS Albion exercised her rights for freedom of navigation in full compliance with international law and norms.”
A source told Reuters that the Albion did not enter the territorial seas around any features in the hotly disputed region but demonstrated that Britain does not recognise excessive maritime claims around the Paracel Islands. Twelve nautical miles is an internationally recognised territorial limit.
The Paracels are occupied entirely by China but also claimed by Vietnam and Taiwan.
A spokesman for the Royal Navy said: “HMS Albion exercised her rights for freedom of navigation in full compliance with international law and norms.”
Dr Euan Graham, a Senior Fellow at the Lowy Institute in Australia, said the move followed an earlier passage by a Royal Navy ship through the Spratly Islands.
He said it was a clear indication of Britain’s support for the US, which has said it would like to see more international participation in such actions.
"Also, the fact that Albion was coming from Japan and on her way to Vietnam gives the signal a sharper edge to China," he told The Telegraph.
The Albion is one of three Royal Navy ships deployed to Asia this year, along with HMS Argyll and HMS Sutherland.
"The UK has impressively deployed three Royal Navy surface ships to Asian waters this year, after a long gap between ship visits, to this part of the world," he added.
Gavin Williamson, the Defence Secretary, said in June that deployment of the three ships was intended to send the “strongest of signals” on the importance of freedom of navigation.
Dr Graham said "the bigger test of UK commitment to regional security in the Indo-Pacific is about the consistency of its military presence into the future".
"The Royal Navy is making encouraging noises about sending assets to participate in FPDA (the Five Power Defence Arrangement) exercises as well as forward basing in future."
The FPDA is a regional security institution between Australia, Malaysia, New Zealand, Singapore and the United Kingdom.
China’s claims in the South China Sea, through which some $3 trillion of shipborne trade passes each year, are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.
Both Britain and the United States say they conduct FONOP operations throughout the world, including in areas claimed by allies.
The British Navy has previously sailed close to the disputed Spratly Islands, further south in the South China Sea, several times in recent years but not within the 12 nautical mile limit, regional diplomatic sources have said.
FONOPs, which are largely symbolic, have so far not persuaded Beijing to curtail its South China Sea activities, which have included extensive reclamation of reefs and islands and the construction of runways, hangars and missile systems.
Beijing says it is entitled to build on its territories and says the facilities are for civilian use and necessary self-defence purposes. China blames Washington for militarising the region with its freedom of navigation patrols.
Foreign aircraft and vessels in the region are routinely challenged by Chinese naval ships and monitoring stations on the fortified islands, sources have said previously.
In April, warships from Australia – which like Britain is a close US ally – had what Canberra described as a close "encounter" with Chinese naval vessels in the contested sea.