The UK Independence Party (UKIP) gained significant support at the election, gaining 12.6 percent of the share of the vote, although only returning one lawmaker to the House of Commons.
Cameron had hoped to broker a deal with his European counterparts at the upcoming summit on 17/18 December. However, he told journalists on a visit to the Bulgarian capital Sofia: "We need time to ensure each issue is properly addressed, because what matters most is getting the substance right and this is a large, bold and wide-ranging agenda.
"It is difficult. We're not going to agree it in one go, so I do not expect to reach agreement at this December summit, but we won't take our foot off the pedal. We will keep up the pace on negotiations and we will use this summit to focus minds and to work on solutions in the toughest areas," Cameron said.
Central to Cameron's renegotiation strategy is a four-year delay on in-work benefits and child benefits for EU migrants working in Britain — a stance opposed by some of his EU colleagues, including Poland.
'Significant Reforms'
Cameron phoned German Chancellor Angela Merkel Thursday to discuss progress on renegotiation. Merkel is as anxious to keep the UK inside Europe as Cameron, Britain being the second biggest economy in the EU and one of its two top military powers.
PM spoke with German Chancellor Angela Merkel earlier today. Full readout: https://t.co/GeJHaT1NQR #EUreform
— No. 10 Press Office (@Number10press) December 3, 2015
However, speaking in Sofia, Cameron conceded the December meeting would not see a resolution to the UK's position in Europe. "There are significant reforms that we are seeking. We need fundamental, legally binding and irreversible changes," he said.
"The scale of what we are asking for means we will not resolve this easily. We need time to ensure each issue is properly addressed because what matters most is getting the substance right.
"This is a large, bold and wide-ranging agenda and it is difficult. We are not going to agree it in one go so I do not expect to reach agreement at this December summit but we won't take our foot off the pedal," Cameron said.
Cameron has promised the In/Out referendum before the end of 2017, however — with French and German elections due that year, which would dominate the EU agenda — Cameron is likely to call one in mid-2016.
