Speaking to the European Parliament in Strasbourg this Tuesday, Juncker also insisted that Britain cooperate with the EU during the delay, which he did not expect to be renewed after October 31st, and be treated as a full member.
Juncker, who heads the 28-nation EU's executive arm, told the parliament "the future of our union is not Brexit."
This is the EU parliament's final session before EU elections taking place from May 23rd to 26th.
"Brexit cannot and will not hinder our progress," Juncker added, saying the delay agreed upon at a summit last week will mean Europe will not be forced to discuss Brexit every week.
He referred to key decisions ahead on a new multi-year EU budget, international trade, and the selection of new European leaders, including his replacement.
EU expectations from London
Echoing terms of the delay the EU granted Britain at a summit last week, Juncker again underlined Britain's role in the months or weeks ahead.
"We are requesting there will be a loyal and responsible cooperation," he said, switching to English from a speech in French and German.
But Juncker also said Britain must be treated as a full member as long as it stays in the block. "We don't have member states of second rank."
Donald Tusk, president of the European Council, underlined Juncker's points on Britain's role ahead and said the delay allowed Europe to focus on other priorities like trade with the United States.
European leaders agreed with Britain last Thursday to delay Brexit by up to October 31st, saving the continent from what could have been a chaotic no-deal departure on April 12th.
The April 12th deadline was a delay from an original deadline of March 29th.
Options for the UK
The EU granted the delays after Prime Minister Theresa May failed three times to get her parliament to adopt the divorce deal she struck in Brussels in November.
The deal also means that, if London remains in the EU after May 22nd, British voters will have to take part in European elections -- or crash out on June 1st.
Juncker underlined the options facing Britain.
"The United Kingdom can also revoke its request to leave, but it is not my working hypothesis," Juncker said. "Nor is it my working hypothesis that there will be an other extension beyond October 31st."