Former European Commission president Mr Barroso has denied he lobbied his former colleagues for his new employer Goldman Sachs.
Following the publication of a report from the European Ombudsman, an independent association investigating into EU institutions, he wrote on Twitter: “I have not and will not lobby EU officials”.
The papers claim that his successor at the Commission, Mr Juncker, had failed to scrutinise whether the Portuguese’s new job undermined the trust the public has in the European Union.
Mr Barroso joined US bank Goldman Sachs in July 2016, less than two years after leaving his role in Brussels.
His decision sparked the fury of Mr Juncker who, although said he could not obstruct his decision, criticised Mr Barroso for taking a job with a major bank after the financial crisis.
Mr Barroso said Ombudsman Emily O’Reily made no “legal assessment” of his duties for the American bank and also stated he did not oppose her call for a new review by the Commission’s ethics committee which, in 2016, found no reason to object to his new nominee.
The former Portuguese prime minister accused Ms O’Reilly of mounting a “thinly veiled ad personam political attack” against him.
Ms O’Reilly replied dismissing Mr Barroso’s account that his meeting in October with Jyrki Katainen was a private matter because Mr Katainen registered it in the EU public lobbying record as a meeting with “Goldman Sachs”.
Mr Katainen was at the time Mr Barroso’s former member of the Commission, whereas today is the vice president under Mr Juncker.
The scandal is hitting Brussels while Mr Jucker is being urged to explain his decision to give a promotion to his chief-of-staff Martin Selmayr.
Last month Mr Juncker took everybody by surprise by appointing Mr Selmayr to the EU Commission’s Secretary-General role.
The decision has sparked corruption claims as nobody else has been interviewed for the job, and was criticised by lawmakers from across the political spectrum, including Jucker’s conservatives.
The affair has grown so much in proportion that it has become known in the EU capital as “Selmayrgate”.
Guy Verhofstadt, the European Parliament’s Brexit representative, described the move as a “misstep” that was “damaging for Europe as a whole”.
The EU insists due process was followed when promoting Mr Selmayr.