Ma Zhengang, a former Chinese ambassador to Britain, said it was good for China’s diplomats to see the official directly responsible for diplomatic affairs being promoted as it highlighted the importance the top leadership placed on foreign affairs.

“It’s recognition of Yang personally and the entire foreign policy establishment in the cause of China’s rise to global importance,” he said.

“We are seeing an unprecedented transition of China’s role, which will not be confined to domestic interests but demonstrate more interest in having a greater say on global issues.”

Despite having reached retirement age, 67-year-old Yang, a former foreign minister and renowned expert on the United States, stands a good chance of becoming a vice-premier with responsibility for foreign policy issues at the cabinet reshuffle in March.

His two predecessors Tang Jiaxuan and Dai Bingguo, both former state councillors, failed to land seats on the Politburo, which was widely seen in diplomatic circles as proof of the foreign ministry’s diminishing influence in shaping foreign policy.

Although he has spent the past 10 years as a member of the Central Committee, Yang has not had access to the highest ranks of power. And while he is not generally viewed as part of Xi’s inner circle, having a seat on the Politburo will undoubtedly give him more influence and better access to the president’s ear.

Bonnie Glaser, of the Washington-based Centre for Strategic and International Studies, said: “The move may signal a recognition that foreign affairs are of growing importance to advancing Chinese interests and should no longer be assigned a lower position.”

Evan Medeiros, managing director of political risk consultancy Eurasia Group and the top Asia adviser to former US President Barack Obama, called Yang’s promotion “a historic development for China’s foreign policy”.

“[It] shows Xi’s elevation of foreign affairs work and his desire for China to play a leading role in Asia and globally,” he said.

Known for his personal connections to the Bush family, Yang has played a pivotal role in helping steady often bumpy Sino-US relations, especially during sensitive periods over the past decade.

He was active in the lead-up to the party congress, and took credit for establishing a personal rapport between Xi and US President Donald Trump, and bringing an end to China’s 10-week border stand-off with India.

Yang Jiechi is also believed, with the help of his younger brother Yang Jiemian, a respected strategic affairs scholar and former president of the Shanghai Institute of International Studies, to have coined one of Xi’s favourite catchphrases – the “new type of major power relations”.

He was also one of the first senior Chinese government officials to publicly pledge allegiance to Xi when he lavished praise on the “core leader’s” thoughts on diplomacy in an article in January.

Alexander Gabuev, a senior fellow at the Carnegie Moscow Centre, said Yang’s promotion to the Politburo should not come as a surprise.

“It’s a sign that foreign policy requires more attention from the top leadership, and needs a curator on the Politburo era,” he said.

“In the age of global turbulence, relationships with key powers, such as the US, EU, Russia, India, Japan and others, as well as the ability to navigate increasing numbers of regional crisis situations, like North Korea and the South China Sea, are vital to the growth of the Chinese economy and domestic stability.

“China’s participation in globalisation has led to increased dependence on the external environment, and since many issues, like trade, are intertwined with geopolitics, China needs a skilled diplomat with full trust from Xi Jinping to help navigate this complicated landscape,” he said.

Both Medeiros and Gal Luft, a co-director of the Washington based think tank Institute for the Analysis of Global Security, said Yang’s promotion was also evidence that earlier rumours that Xi had doubts about Yang were overstated or even mistaken.

“He must be someone who is fully trusted and fully backed by Xi,” Luft said.

Other observers said that elevating Yang was a sign that the top leadership in Beijing wanted to ensure continuity on foreign policy, which has seen a landmark shift in the past decade towards increased assertiveness amid soaring international scepticism and concerns about the intentions behind China’s rise.

Shi Yinhong, of Beijing’s Renmin University, said: “China is expected to markedly step up its role in global political and economic governance, which is aimed at increasing [its] strategic influence in the region to counter the US and Japan.”

Dibyesh Anand, a China expert at Westminster University in London, said Yang had extensive hands-on diplomatic experience with the US, Japan and India, and if he maintained that approach it would augur well for smooth relations between China and other major powers.

“However, Xi’s personality ensures it is he who will be paramount when it comes to not only domestic politics but international relations, so Yang’s role will be to facilitate Xi’s vision of China as a great power diplomatically,” he said.

Chinese diplomats said that with Yang’s promotion and the unveiling of China’s new leadership line-up at the end of the party conclave on Wednesday, a generational reshuffle of the country’s top diplomats was expected to begin soon.

Additional reporting by Liu Zhen