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Belgium's former prime minister said allegations that the US has spied on leaders of France and Germany were "a scandal".
Guy Verhofstadt told the BBC's Today programme:
European heads said a "lack of trust" would undermine their cooperation with the US in the fight against terrorism, amid reports that the National Security Agency monitored leaders' phone conversations.
Addressing "deep concern" over the issue, an EU Council statement said the partnership between Europe and the US was vital in fighting terrorism but "must be based on respect and trust".
It added: "A lack of trust could prejudice the necessary cooperation in the field of intelligence gathering."
David Cameron has arrived for the second day of the EU summit in Brussels, which was due to focus on cutting business red tape but is set to be overshadowed by the allegations that the US monitored the mobile phones of 35 world leaders.
European leaders have reacted with anger at a European summit after it emerged that the US National Security Agency had monitored their telephone conversations after obtaining their numbers from an official in another government department.
The revelation has led to widespread condemnation, and the French and German governments have demanded talks with the United States by the end of the year to resolve the dispute and attempt to restore trust.
French president Francois Hollande said:
German chancellor Angela Merkel spoke of her upset at the alleged phone monitoring, saying:
European leaders have united in concern over claims the United States monitored the mobile phone of German Chancellor Angela Merkel, and other world leaders.
Earlier today, Mrs Merkel said "transparency" is needed between the US and Europe and "spying on friends is not acceptable at all"
ITV News' Europe Editor James Mates reports for an EU summit in Brussels.
German chancellor Angela Merkel arrived at an EU summit in Brussels today in a car with a ‘007’ number plate amid a row with the US over alleged spying.
Mrs Merkel contacted President Obama after receiving information the US may have spied on her mobile phone.
Speaking today, Mrs Merkel said she "made it clear to the president of the United States that spying on friends is not acceptable at all".
Classified documents provided by former computer contractor Edward Snowden have revealed that the US National Security Agency monitored the phone conversations of 35 world leaders.
The surveillance is understood to have begun after the NSA was given the numbers by an official in another US government department.
According to a report in The Guardian, the confidential memo shows that that the NSA encouraged senior officials in its "customer" departments to share their "Rolodexes".
Customer departments would have included the White House, State and the Pentagon, allowing the agency to add the phone numbers of leading foreign politicians to surveillance systems.
EU leaders have been giving their reaction to accusations to German and French accusations that the United States has been running spying operations in their countries.
Belgium Prime Minister Elio Di Rupo said: " We cannot accept this systematic spying, whatever it may be. We need to take European measures."
Swedish Prime Minister Fredrik Reinfeldt said surveillance should not be used to "listen to each other, when we look at elected leaders."
Finland's Prime Minister Jyrki Katainen said: "We have to get clarification of what has happened and we also need a guarantee that this will never happen again if it has happened."
"This is serious. I will support her (Merkel) completely in her complaint and say that this is not acceptable. I think we need all the facts on the table first", Dutch Prime Minister Mark Rutte said.
German Chancellor Angela Merkel has said she had made it clear to US President Barack Obama that spying on allies is "not acceptable at all"
Speaking in Brussels, she said: "I have, since we have been speaking about the NSA, made it clear to the president of the United States that spying on friends is not acceptable at all.
"I said that when he was in Berlin in July and also yesterday in a telephone call".
Yesterday, it emerged that Mrs Merkel had contacted the president after receiving information the US may have spied on her mobile phone.
The US has denied monitoring the chancellor's communications.
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