2007: Barroso: An insider's guide to the Lisbon Treaty

Barroso said Brown deliberately arrived late to eschew the official signing ceremony and group photo with the other heads of state and government at Lisbon's 15th-century Jerónimos Monastery.

"In fact, he signed very discreetly the treaty with me and one or two other witnesses and without the media. For me the fact that Gordon Brown, someone who may be considered pro-European was not ready to sign publicly the Lisbon Treaty, that is a clear, clear signal of what could come later," said Barroso.

The Lisbon Treaty forms the constitutional basis of the European Union, amending two other treaties to meld cross-border rights and freedoms. It entered into force at the end of 2009.

Among the novelties it introduced at the institutional level was a more powerful European Parliament, a new European Commission foreign policy branch, and a new European Council presidency.

But it also included a new option for a member state to leave - which the UK subsequently did, following the 2016 referendum that has since led to years of protracted and painstaking negotiations between London and Brussels.

Barroso first became European Commission president in 2004, the same year 10 states joined the union.

He was then reappointed in 2009, allowing him to experience the impact of the Lisbon Treaty on the EU and its institutions first hand.

The German EU presidency, in the lead up to the treaty's signing, had helped create a political momentum, following the Berlin declaration over the 2007 summer months.

But negative referenda results in France and the Netherlands had initially threatened to derail it. Ireland was also at odds.

"I remember going there [Ireland] at least twice, engaging in debates with society and different groups. Frankly, one of the most difficult groups was, it is bizarre to say, but it was the meat producers and exporters," said Barroso.

Barroso said Ireland was wary of having its beef industry compete with other huge exporters, like Brazil. That issue was eventually settled, with Dublin's decision to support the treaty a factor that weighed in on Poland.

In his conversations with Poland's president at the time, the late Lech Kaczynski, Barroso said he saw a leader who was culturally conservative, but also very much someone who believed in an integrated European Union.

"I remember speaking about the possibility of the European army, he said to me, 'if one day there is a European army, I would like Poland to be the first to contribute, to the first division', so someone who says this I don't consider anti-European at all," said Barroso.

As for signing off on the Lisbon Treaty itself, Barroso said Kaczynski was ready to support it, but did not want to appear to be putting undue pressure on a smaller country like Ireland.

"And I asked him if he was ready to ratify the treaty because there was some delay and he said to me 'look, I am ready to do it today, if you tell me that Ireland does not see a problem'," said Barroso.

Ireland held a second referendum, backing the treaty, in October 2009. Poland's parliament then ratified it soon afterwards.

The changes at the EU institutions were profound. Not everything went smoothly, as a turf war over foreign policy broke out between the European Commission and the European Council.

Barroso had told Brown to send him a woman to take up the role of the EU's foreign policy chief, more formally known as the high-representative. Brown sent Catherine Ashton.

"I wanted to have a more gender-balanced commission, I made with him the same kind of 'soft blackmail' I made with several prime ministers, telling them look if you send me a man I will give your commissioner a very secondary portfolio, but if you send a woman I can give her an important portfolio," said Barroso.

Barroso then also appointed Ashton as the vice-president of the European Commission, creating a dual role between the EU institutions. She then helped set up the new European External Action Service.

"If I want to be honest, there were some turf issues," said Barroso, noting not everyone at the commission was happy about delegating foreign affairs to another body.

Big EU states also wanted to keep Ashton close, and away from the commission, he said, noting some friction as well from the new European Council president.

"Once again being very honest, many people in the European Commission were looking at this position with suspicion," he said.

Some commission insiders saw themselves as the only institution that had a permanent full-time position on EU matters - while the European Council president post was a two-and-a-half year appointment.

Despite the teething problems, Barroso describes the EU foreign policy branch as a "silent revolution", given the current role of European embassies, missions and delegations around the world.

"I think we have to be proud of the work done with the Lisbon Treaty," Barroso concluded.