In an article published in The International Herald Tribune on Saturday, Rehn says the EU has to take into account the concerns of those who believe that the enlargement process is too fast and be cautious in assuming new obligations, but he also stresses that the EU was founded on the principle of sticking to one's word.
"This means that Bulgaria and Romania will join the Union in 2007, if they fulfil the conditions. It also means that accession negotiations will start with Croatia and Turkey once the two countries meet the strict criteria."
"I would like to start negotiations with Croatia, but that will only be possible once the country is cooperating fully with the United Nations' War Crimes Tribunal in The Hague," Rehn says.
He admits that the atmosphere in Europe is gloomy after France and the Netherlands voted against the EU Constitution, but adds that the issue of enlargement was not the reason for the rejection of the constitutional treaty.
He dismisses requests by some politicians to slow down or even stop the enlargement process, saying that one should not draw the wrong conclusions and think that the enlargement process was the reason for the negative referendum votes.
"It would be irresponsible to disrupt a valuable process that is helping to build stable and effective partners in the most unstable parts of Europe," in western Balkans and Southeast Europe, where the prospect of membership has been a strong impulse for reforms, Rehn says.
He goes on to say that the enlargement process is very successful, and adds that it has proven to be one of the most important instruments of Europe's security.
He warns that possible indecisiveness regarding the prospect of EU membership for western Balkan countries could erode the positive influence of that prospect just when the region is entering a difficult period of talks on Kosovo's future status.
"The European perspective for the whole region is the key to finding a sustainable solution for Kosovo."