David Cameron will miss the first meeting of European Union leaders since submitting his demands for reform - with Dutch prime minister Mark Rutte representing the UK.
The Prime Minister is unable to attend the informal heads of government meeting in Valletta because he will be welcoming Indian counterpart Narendra Modi in London.
Ukip leader Nigel Farage said it was the "greatest surrender" yet of British sovereignty.
The meeting of EU leaders was called by European Council president Donald Tusk to follow the international migration summit in the Maltese capital.
Mr Cameron discussed his proposals for reform with fellow EU leaders on Tuesday in talks in the margins of the Valletta summit, but will return to London before the informal meeting of heads of government.
Although Home Secretary Theresa May will be in Valletta for the closing stages of the summit on the migration crisis, she is unable to attend the meeting of leaders.
A Downing Street spokeswoman said: "It's an informal council and so we don't expect decisions to be taken. And as the Prime Minister will be hosting prime minister Modi for bilateral discussions in London, he has asked prime minister Rutte to represent him as necessary."
A Government source stressed the situation was not unusual for an informal council meeting, and indicated that the Irish are being represented by the Danes while the Czechs would represent Poland.
But Mr Rutte's role will raise eyebrows after he warned that a British exit from the EU would be a "killer" for the City of London and leave the UK as a "mid-sized economy" in the mid-Atlantic.
In an interview with Bloomberg News earlier this week Mr Rutte said: "It would no doubt take away a lot of economic potential, and be a killer for the London financial centre."
A British exit from the EU would mean the UK becoming "a mid-sized economy in the middle of the Atlantic Ocean, in neither America nor Europe".
Ukip leader Mr Farage said: "This is the greatest surrender of British sovereignty yet.
"Not only are we in a small minority at the European Council, but now we are being represented by a foreign country."