British Prime Minister David Cameron found his reform agenda sidelined at a tense EU summit as leaders warned he faces tough negotiations ahead of a referendum on whether to leave the bloc.

As he walked into the Brussels summit dominated by Greece and migration, Cameron said it marked a "significant milestone" in Britain’s bid to renegotiate ties with the European Union before a referendum due by the end of 2017.

European leaders discussed his proposals for just a few minutes at dinner between fraught discussions on migration before agreeing to move to the next stage in the process.

This involves detailed technical talks after the broad discussions which Cameron has so far had on the issue with his fellow 27 EU leaders.

It is thought the issue will get only a short mention in Friday’s final communique.

EU leaders are focused on trying to keep Greece in the eurozone and halting an influx of migrants from Middle East and African trouble spots, meaning substantive talks on Britain’s desired reforms will have to wait.

Nevertheless, EU President Donald Tusk warned that only changes which were "safe" for Europe would be considered.

"One thing should be clear from the very beginning — the fundamental values of the EU are not for sale and non-negotiable," Tusk said.

Martin Schulz, president of the European Parliament, also fired a shot across the bows of Cameron, who has insisted the changes he wants require treaty change.

"I think treaty change is quite difficult and the UK government should not only exclusively focus on treaty change," he said.

British officials accepted yesterday that any treaty change may not be implemented by the referendum but that Cameron would secure legally binding pledges that EU laws would be altered to include the reforms.