Much earlier this morning, I asked the European Commission President at his press conference at the G7 summit if he could ever work with Boris Johnson after the comments the former London Mayor made about the EU and Hitler.
Jean-Claude Juncker responded, and failed to hide his contempt for the prominent pro-Leave campaigner in the EU referendum.
"It's time for [Boris Johnson] to come back to Brussels," he told me, "in order to check...if everything he is telling the British people is in line with reality. I don't think so."
Tonight Boris Johnson has responded.
In a politely worded letter, he accepts the invitation to come to Brussels, and says: "I would very much welcome the chance to explain how [leaving the EU] would be of benefit both to this country and the rest of the EU."
And then he turns the tables on his European nemesis.
Unlike other recent late night responses from Vote Leave, this letter shows no sign of animosity.
Previous late evening responses to ITV's announcement of a TV programme involving Ukip leader Nigel Farage and to the Institute for Fiscal Studies report into Brexit have been criticised for the tone and put Vote Leave on the defensive in the following hours.
I suspect Mr Juncker will not take up Boris Johnson on his invitation.
Imagine the chaotic scenes as a forceful campaigner for Brexit toured parts of Britain with the chief bureaucrat in Brussels.
And Mr Juncker would also be left in the awkward position of having to explain what his chief of staff meant today when he tweeted about a 'horror scenario' of Boris Johnson being prime minister in the UK and Donald Trump being president in the US.
And looking beyond the referendum, what happens to his working relations with the EU if Boris Johnson becomes a government minister?
It would be tricky, to say the least.
- Boris Johnson's response in full: