A confidential assessment by the Royal United Services Institute has identified a previously undetected pattern of naval activity in the South China Sea, prompting warnings from British defence analysts that the region is entering a new phase of strategic competition. The report, which draws on satellite imagery and signal intelligence, documents a 40 per cent increase in the number of unidentified underwater contacts near the Second Thomas Shoal since October 2023. Analysts believe these movements represent a systematic effort by Beijing to map the seabed in preparation for potential cable-laying or submarine patrols.
The Second Thomas Shoal, part of the Spratly archipelago, is a frequent flashpoint between Chinese and Philippine forces. Manila maintains a small garrison on a deliberately grounded warship, the BRP Sierra Madre, which has been resupplied under regular escort from the Philippine navy. The RUSI assessment, however, notes that Chinese vessels have recently begun conducting hydrographic surveys at depths of more than 200 metres, far beyond the shoal’s immediate vicinity. This suggests a shift from tactical obstruction of Philippine operations to a broader effort to contest control of the underwater domain.
Dr. Jonathan Shaw, a former British naval attaché to Beijing and co-author of the report, described the development as a “critical threshold moment.” He noted that the South China Sea is a vital conduit for global trade, with an estimated 30 per cent of the world’s maritime traffic passing through its waters. “What we are seeing is not a random increase in fishing activity, but a deliberate, intelligence-driven campaign. The People’s Liberation Army Navy is building a comprehensive picture of the sea floor, which would give it an asymmetric advantage in any future confrontation,” Shaw said.
The warning comes amid a broader reassessment of Indo-Pacific strategy by the United Kingdom. In 2021, HMS Queen Elizabeth led a carrier strike group through the region, and the Royal Navy has since maintained a persistent presence with offshore patrol vessels and frigates. However, the RUSI analysis suggests that such deployments may be insufficient to deter Beijing’s strategic consolidation. The report calls for the UK to deploy dedicated survey vessels and sonar buoys to monitor Chinese underwater activity, and to share data with allies including Australia, Japan and the United States.
A Foreign Office spokesperson declined to comment on intelligence matters, but reaffirmed the UK’s commitment to the rule of law and freedom of navigation in the South China Sea. “We will continue to support the peaceful resolution of disputes in accordance with international law, including the 2016 arbitration ruling,” the spokesperson said.
The Philippine government confirmed that it had received a confidential briefing from British officials in late February. A Philippine foreign ministry official, speaking on condition of anonymity, said the assessment reinforced Manila’s own concerns about Chinese behaviour in the area. “We are seeing new types of vessels, new patterns of movement. It is as if the PLA Navy is conducting a dress rehearsal for future operations,” the official said.
The Chinese foreign ministry dismissed the report as “baseless alarmism” and accused Britain of “interfering in regional affairs”. In a statement, ministry spokesperson Zhao Lijian said China’s activities in the South China Sea were lawful and transparent, and that “outside forces have no business stirring up trouble.”
Nevertheless, the RUSI assessment has already prompted a review of naval posture by several Nato countries with Pacific interests. A senior French naval officer, speaking at a closed-door conference in Singapore, acknowledged that existing intelligence-sharing mechanisms were “inadequate” to the scale of the challenge. Discussions are under way to create a dedicated maritime security cell in the region, modelled on the Nato alliance’s approach to the Atlantic.
The report concludes with a stark warning: if left unchecked, the new pattern of underwater activity could allow China to establish a de facto surveillance and denial capability across the entire South China Sea within the next five years. The UK, along with its allies, must now decide whether to match Beijing’s long-term commitment to the contest for undersea dominance.
