Stefan A. Halper, the FBI source who assisted the Russia investigation and is at the centre of a standoff between congressional Republicans and the Justice Department, is a well-connected veteran of past GOP administrations who convened senior intelligence officials for seminars at the University of Cambridge in England.

In the summer and fall of 2016, Mr Halper, then an emeritus professor at Cambridge, contacted three Trump campaign advisers for brief talks and meetings that largely centred on foreign policy, The Washington Post reported last week.

At some point that year, he began working as a secret informant for the FBI as it investigated Russia's interference in the campaign, according to multiple people familiar with his activities.

The Post had previously confirmed Halper's identity, but did not report his name following warnings from US intelligence officials that exposing him could endanger him or his contacts. Now that he has been identified as the FBI's informant by multiple news organisations, including the Wall Street Journal, New York magazine and Axios, The Post has decided to publish his name.

Mr Halper, 73, declined to comment. The FBI declined to comment.

Mr Halper's contacts with Trump advisers around the start of the FBI's counterintelligence have come under scrutiny in recent weeks by House allies of President Donald Trump. Late last month, House Intelligence Committee Chairman Devin Nunes, R-Calif., issued a subpoena to the Justice Department requesting all documents related to the FBI informant.

In recent days, Mr Trump has seized on the reports about Mr Halper's role in the Russia probe, suggesting in tweets that the FBI improperly spied on his campaign. There is no evidence to suggest Mr Halper was inserted into the Trump campaign, but he did engage in a pattern of seeking out and meeting three Trump advisers.

On Monday, the conflict was defused – at least temporarily – with the announcement that White House Chief of Staff John Kelly plans to convene a meeting between top law enforcement officials and GOP congressional leaders to "review highly classified and other information" the lawmakers have requested about the source.

Mr Halper's connections to the intelligence world have been present throughout his career and at Cambridge, where he ran an intelligence seminar that brought together intelligence officials of past and present.

In 2014, Mr Halper, along with Richard Dearlove, the former head of Britain's foreign intelligence service, sponsored a meeting of the seminar that drew then-Director of the Defence Intelligence Agency Michael Flynn, who would go on to serve as Mr Trump's first national security adviser.

Mr Halper taught international affairs and American studies at Cambridge from 2001 until 2015, when he stepped down with the honorary title of emeritus senior fellow of the Centre of International Studies, according to a spokesman for the university.

Since 2012, Mr Halper has had contracts with the Defence Department, working for a Pentagon think tank called the Office of Net Assessment. According to federal records, ONA has paid Mr Halper more than $1 million for research and development in the social sciences and humanities.

The funds did not go solely to Mr Halper, who hired other academics and experts to conduct research and prepare reports, according to US government officials.

"He thinks well. He writes critically. And he knows a lot of people whose insights he can tap for us as well," one US government official said.

Mr Halper's first wife was the daughter of the prominent former CIA analyst Ray S. Cline, who worked alongside President John F. Kennedy during the Cuban missile crisis in 1962 and mentored Mr Halper, introducing him to associates in the intelligence and political communities, according to numerous people familiar with their relationship.

After earning his doctorate from the University of Oxford in 1971, Mr Halper quickly ascended, serving on the White House domestic policy council for President Richard M. Nixon and then in the Office of Management and Budget before being tapped as an assistant to President Gerald Ford's chief of staff. According to a document from Ford's presidential library, part of Mr Halper's job was assessing domestic political candidates, such as Jimmy Carter, for high-ranking staffers in the West Wing.

Mr Halper later worked for Sen. William Roth, R-Del., before joining the George H.W. Bush campaign in 1980 as national policy development director and then working for the Reagan-Bush campaign as national director of policy coordination. In the Reagan administration, he served as deputy assistant secretary of state for politico-military affairs, according to his biography.

After the 1980 race, Mr Halper was caught up in a scandal around alleged political spying. Aides to Mr Reagan, including Mr Halper, were accused of having spied on Mr Carter's campaign and obtaining private documents that Mr Carter was using to prepare for a debate. Some Reagan White House officials later alleged that Mr Halper had used former CIA agents to run an operation against Mr Carter. Mr Halper called the reports at the time "absolutely false" and has long denied the accusations.

Between 2000 and 2001, Mr Halper contributed more than $85,000 to George W. Bush's first presidential bid and the Republican National Committee, according to campaign finance records. Most friends describe him as a moderate Republican who is hawkish on China and deeply committed to US institutions, having worked for years inside and around the federal government.

Late in his career, Mr Halper emerged as a vocal critic of President George W. Bush's interventionist foreign policy. During classes at Cambridge, he often raised questions about Mr Bush's decisions and embraced a traditional Republican approach to foreign policy that emphasised long-standing Western alliances and limited foreign intervention, as witnessed by a Post reporter who studied under Mr Halper in 2009. His 2004 co-written book, "America Alone: The Neo-Conservatives and the Global Order," was critical of the Bush administration's approach to the Iraq War.

Mr Halper has spent considerable time focused on China over the past decade, publishing "The Beijing Consensus: Legitimizing Authoritarianism in our Time" in 2010 that warned of China's attempts to build an economic and industrial presence in Africa and elsewhere as a threat to global stability.

"Stef" – as Mr Halper is called by people who know him – was also widely known at Cambridge as a gregarious gatherer of students and academics at his apartment in the city, along with his wife. He frequently hosted dinners with visiting students and scholars from around the world where – over wine and cheese from the local market – he would share colourful stories about his work for American presidents and the US government and stir debates about the issues of the day.