He was speaking at the start of a two-day forum on Southeast Europe on the road to the EU which the German Bertelsmann foundation organised in Zagreb.
Sanader said the forum was taking place at an excellent moment because Croatia was at the Union's doors, but also because of the situation the Union found itself in after France and the Netherlands rejected the European Constitution.
"This opportunity should be used and enlargement should not be abandoned. It should be persevered on because that goal is too important to be given up quickly," Sanader said in his address.
According to Sanader, criteria must be met in order to enter an elite club such as the EU. He said Croatia must do so too, including by cooperating with the Hague war crimes tribunal.
The PM said Croatia had proved over the past two years that it was cooperating, having extradited a dozen indictees, and that there remained one outstanding issue.
"I spoke very openly with chief prosecutor Carla del Ponte yesterday and we agreed that the Action Plan really moved things forward," Sanader said, adding that del Ponte positively assessed Croatia's activities to close the outstanding issue.
Sanader said that what was happening in Croatia was helping its neighbours because Croatia served as a model.
"Croatia deserves to take part in the 2009 elections for the European Parliament as a full member," said the PM.
In his address, Austrian PM Wolfgang Schuessel said it was clear that the next EU enlargement round must include Southeast Europe, but that internally EU institutions must also consider how they operated.
Schuessel said the progress of the region's countries was spectacular and that this must not be disregarded.
He recalled that for the first time in history, all countries in the region had a uniform goal -- admission to the EU.
Regarding Croatia, Schuessel said Zagreb had done a lot, assuming with Ljubljana the role of forerunner in the entire region.
Schuessel said he personally disagreed with an entire nation being held back because of one war crime suspect, saying that an individual could not be behind a collective.
The chairman of the Zagreb forum, Bertelsmann Executive Board member Werner Weidenfeld, said representatives of 20 countries, 10 governments and six international organisations gathered in Zagreb. He added the forum was organised because bilateral relations between the region's countries and Brussels and other European capitals were not enough because Europe should have a uniform vision of the entire area.
Speaking of the region's countries' prospects of joining the EU, Weidenfeld said the question had never been whether to expand the Union but how to do it because the Union was founded on the notion of unification of all European democracies.
Weidenfeld said that if Europe wanted stability and peace, it could not leave a huge part of its territory out, notably because important reforms had been carried out in all countries of the region.
Speaking of Croatia, he said that the Bertelsmann foundation ranked Croatia 14th among the 120 countries it evaluated in 2003, adding the foundation's latest ranking would put Croatia at the 12th position.
Weidenfeld said the foundation estimated in its strategic document that judging by results so far, the last south-eastern European country to join the EU would join in 2014. He added that the Union would have to offer more actual assistance, not just statements as in the past.