Sky News deputy political editor, Sam Coates, tweets
Citing a government source on the matter, says that the UK government is readying up a response after the EU shot down Boris Johnson's letter with the view:
"We have made reasonable proposal, the EU is not negotiating and instead spending its efforts trying to undermine proposals amongst the EU 27, the reality the backstop has to be removed or else it's no deal."
The remark on undermining the proposal is a bit of a stretch. It's not like they didn't know what the EU's stance is and that hasn't changed since even when Theresa May was in-charge.
If anything else, this continues to show that negotiations later in the week between Johnson with Merkel and Macron are not going to lead to anything.