Polish Prime Minister said he wants a Breturn rather than Brexit in a press conference alongside Sir Keir Starmer on his visit to Warsaw.
Sir Keir has repeatedly ruled out rejoining the customs union or single market but said he wants to deepen ties with Brussels post-Brexit.
However Mr Tusk who was president of the European Council when Britain voted to leave the trading bloc, said he was already mourning the loss of the UK on the day of the referendum.
Standing alongside Sir Keir at a joint press conference, Mr Tusk said: Im sure you will recall when we learned about the results of the Brexit referendum.
My first emotional reaction was to say: I already miss you. I remember our press briefings as if it was yesterday. I already miss you, thats what I said.
This is not just about emotions and sentiments - I am aware this is a dream of mine, that instead of a Brexit we will have a Breturn.
Perhaps Im labouring under an illusion. Id rather be an optimist and harbour these dreams in my heart - sometimes they come true in politics.
During the press conference, Sir Keir did not go into detail about Brexit, but stressed the need for close co-operation among European allies on issues including people smuggling and security.
Sir Keir Starmer and Polish Prime Minister Donald Tusk shake hands after a press conference in Warsaw
Sir Keir smiles as he arrives to the press conference with his Polish counterpart Mr Tusk
Donald Tusk said he dreams of a Breturn with Britain rejoining the EU in the future
He did however exchange warm words with his counterpart, telling Mr Tusk: Poland currently holds the presidency of the EU Council.
I want to congratulate you, Donald, on making European security the focus of your presidency, because this is the issue of our time.
Sir Keir travelled to Poland after meeting Ukrainian President Volodymyr Zelensky in Kyiv on Thursday, where he vowed the UK will play our part in guaranteeing the countrys security following any peace deal with Russia.
In Warsaw he discussed a new bilateral defence pact with Mr Tusk, which the Polish prime minister said he hopes will be ratified this year.
Speaking about the deal on Friday, Sir Keir said: This includes deepening ties between our defence industries.
The UK has secured £8 billion of defence deals in Poland over the last three years alone, and were going further today, opening a new joint programme office in Bristol to deliver our £4 billion partnership, to deliver the next generation of air defence systems to Poland.
He added: We share an unbreakable commitment to Nato and an unbreakable commitment to Ukraine.
Both leaders hope the treaty will help the UK and Poland work together more closely to protect Europe from Russian aggression and co-ordinate a response to tackle people-smuggling gangs.
Keir Starmer and his wife Victoria lay a wreath during a visit to the Memorial And Museum Auschwitz-Birkenau
The Prime Minister described it as utterly harrowing, and reiterated his determination to fight the poison of antisemitism
In a readout of their bilateral meeting, Downing Street said the two men had warm and productive talks on issues such as migration, energy and tackling shared threats - including Putins ongoing aggression.
Sir Keir also met Polish President Andrzej Duda, with whom he agreed that Ukraine can count on the support of the UK and Poland for as long as it takes, according to a Number 10 spokesperson.
Earlier on the trip, the Prime Minister also visited Auschwitz, a place he described as utterly harrowing, and reiterated his determination to fight the poison of antisemitism.
His wife Victoria, who is Jewish, joined him for the visit and was equally moved, Sir Keir said.
The truth that I have seen here today will stay with me for the rest of my life, the Prime Minister said.
So too will my determination to defend that truth, to fight the poison of antisemitism and hatred in all its forms, and to do everything I can to make never again mean what it says, and what it must truly mean: never again.