European Council President Donald Tusk has shared a letter from a six-year-old English girl, featuring an apparently pointed message to Brexit-backing MPs.

"Dear Mr Tusk, I live in Britain. I know we are leaving the EU. But I think we should be friends. From Sophie, aged 6."

Mr Tusk responded in his social media post by writing: "We will always be friends, Sophie."

But the letter also included a drawing of a unicorn, a term which has been used as slang for the unrealistic plans pursued by some UK lawmakers during the Brexit negotiations.

Asked about the meaning of the unicorn reference, European Council replied it did not comment on social media posts, adding they were open to individual interpretation.

European Parliament's Brexit coordinator, Guy Verhofstadt, said the letter put "everything in perspective".

"Sometimes it takes a child to put everything in perspective," Mr Verhofstadt tweeted.

"What a terrible waste #Brexit is."

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Mr Tusk has a history of tongue-in-cheek social media posts on Brexit.

In September, he shared a photo of himself and British Prime Minister Theresa May choosing a sweet treat from a spread of food at a summit in Salzburg.

"A piece of cake, perhaps? Sorry, no cherries," the post read, in an apparent reference to cherry-picking in the process of the UK's negotiations with the EU.

In February, Mr Tusk prompted angry responses when he tweeted there was a "special place in hell" for those who promoted Brexit without a plan to deliver it.

"I've been wondering what that special place in hell looks like, for those who promoted #Brexit, without even a sketch of a plan how to carry it out safely," he wrote at the time.

MPs are set to vote today on whether to block a no-deal Brexit, after Mrs May suffered a humiliating defeat as her EU Withdrawal Agreement was rejected by an overwhelming majority for the second time yesterday. 

MPs voted by 391 to 242, a majority of 149, against the Prime Minister's Brexit deal despite her assurance new agreements had been reached with Jean-Claude Juncker in Strasbourg.