The document was revealed by Aidan O'Neill QC, the lawyer acting for campaigners who want judges to issue an interdict, or injunction, forcing the PM to delay exit day if no agreement is signed off by 19 October.
The legal action taking place in Scotland's highest court therefore asked the judiciary to require Mr Johnson to seek an extension to avoid leaving the European Union without a deal.
Sassoli will also see French President Emmanuel Macron in Paris on Monday and Johnson in London later on Tuesday.
However, his government also acknowledged for the first time on Friday that Johnson will send a letter to European Union asking for a Brexit delay if no divorce deal has been reached by October 19.
In that declaration, he said that not reaching a deal by the UK's scheduled October 31 departure date would be "a failure of statecraft for which we would all be responsible".
Johnson's office did little Friday to quash such talk.
"The government will comply with the Benn Act, which only imposes a very specific narrow duty concerning parliaments letter requesting a delay", she quoted the source as saying. But its also possible hes acting in good faith, and its worth noting that the two sides are still talking.
"The government is making its true position on delay known privately in Europe and this will become public soon".
Court papers have been flying in the face of Johnson's comments since he succeeded Theresa May as prime minister this summer.
The British proposal outlines measures to ease trade on the Irish border but with Great Britain and its Northern Ireland territory leaving the EU customs union.
Or, as Theresa Mays withdrawal agreement proposed, it could leave the union in the sense of no longer having any say in its policies, while being forced to follow majority for as long as the EU saw fit.
Mr Varadkar said he could not fully understand how the United Kingdom envisages Northern Ireland and Ireland operating under different customs regimes without the need for checkpoints.
"There is a likelihood that we derive an agent provocateur... which the govt. makes exhaust of as a pretext to acquiring emergency powers", added the aged politician, who as soon as described Leave voters as "Brexit jihadis".
Dutch Prime Minister Mark Rutte said Friday that Britain's new Brexit proposal could "at best" form a basis for further discussions, but many questions still remain.