European Council President Donald Tusk warned on Thursday (12 November) it would be "very, very tough" to reach a deal on reforms Prime Minister David Cameron has asked for to help keep Britain in the bloc.
Cameron outlined four key demands in a letter to Tusk earlier this week.
>>Read: Cameron sketches out EU reform demands, stays quiet on timing
Responding publicly for the first time, Tusk said in La Valletta: "For sure the requests are tough and this is why the letter was so interesting for me."
Tusk said it would be very difficult to reach an agreement and said there was "no sure guarantee" a deal could be done by December, when EU leaders are due to review Britain's proposals at a summit in Brussels.
"For sure it is a very, very tough game," Tusk, a former Polish prime minister, added.
Tusk said detailed negotiations would begin next week with the other 27 member states invited to "a kind of confessional" to give their views on the changes London is seeking before a planned referendum on whether to leave, which Cameron has pledged to hold by 2017.
The Conservative premier has refused to rule out the possibility of campaigning for 'Brexit' - a British exit from the EU - if his reform proposals are not approved.
The most controversial aspect of Cameron's proposals is a request for the right to restrict welfare and in-work benefits for newly arrived migrants from other European Union states.
Eastern European countries with many citizens working in Britain regard the proposal as discriminatory, and the European Commission has described it as "extremely problematic".
India wants UK in EU
In the meantime, Indian Prime Minister Narendra Modi who is on a visit to Britain said he wanted the UK to stay in the EU.
London rolled out the red carpet for Modi on his three-day visit, the first by an Indian prime minister in nearly 10 years. He will meet Queen Elizabeth II today (13 November) and address a huge rally at Wembley Stadium.
After talks with Cameron, Modi became the first Indian premier to address Britain's parliament, and was feted with a flypast by the Red Arrows aerobatics team, which trailed smoke over London in the colours of India's flag.
Cameron announced the trade deals at a press conference and also revealed that £1.0 billion of rupee-denominated bonds would be traded in London.
"We want to forge a more ambitious, modern partnership, harnessing our strengths and working together for the long term to help shape our fortunes at home and abroad in the 21st century," he said.
The deals are expected to include India buying more BAE Systems Hawk trainer jets -- the same plane used by the Red Arrows -- for its armed forces.
But when Modi was asked whether Britain should remain in the EU, in a referendum called by Cameron and being held by the end of 2017, Modi said: "As far as India is concerned, if there is an entry point to the European Union, that is the UK."