Just a day after European Union chief negotiator Michel Barnier urged his British counterpart to "start negotiating seriously", Mr Juncker echoed the bloc's refusal to discuss the future free trade deal London wants before pencilling in terms for it leaving the EU.
The European Parliament has an oversight role in the negotiations and crucially must approve any final Brexit accord.
The U.K.is set to leave the EU in March 2019 after the majority of the British voted in favor of the country leaving the union via a referendum previous year.
A senior British MEP on Wednesday said Guy Verhofstadt, the European Parliament's point-man on Brexit, had gone "way beyond his pay grade" by suggesting that talks on the U.K.'s exit would not be able to move to the next level in October.
It also raised hopes among some who voted "remain" in last year's referendum on European Union membership that there would be a challenge in parliament to what they say is the prime minister's pursuit of a hard break with the European Union which could end in no deal.
A source close to Corbyn said killing off the bill was not the aim of the party leadership and some pro-EU lawmakers in constituencies where voters supported leaving the bloc might find it hard to scrap laws that are central to Brexit. On the political side, the UK's close ally across the Atlantic might hamper the EU's hopes for getting a hefty Brexit payment; the Trump administration has repeatedly expressed its willingness to strike a trade deal with the UK.
She told reporters on the plane she wants the U.K.to keep the benefits of any trade deal struck between the European Union and Japan after it leaves the bloc. Such a proposal would hardly work for the Tories ruling in London, as, for the UK, Brexit is all about regaining full national sovereignty.
"Negotiations ongoing: (EU leaders) guidelines are designed for serious and constructive negotiations but we need clear United Kingdom positions on all issues", Barnier said in a tweeted message.
Meanwhile, the UK's economy is the slowest among the G7 due to rampant inflation and uncertainty exacerbated by the Brexit process.
The prime minister plans to tell the summit that Britain is willing to hand over a significant amount of money to Brussels as part of Brexit but only as part of a wider deal that'll allow it privileged access to the single market and a customs arrangement during a transitional period.