Speaking at a news conference in parliament on Monday, the former Bulgarian prime minister said the big gains for such parties was "not so much about European politics but more about national policies and a protest vote".
He went on, "The fact that parties like Front National and UKIP, which won more votes in the UK than another other party, can gain such support do so well is very serious and cause for concern. It should sound a warning bell to other parties and send a message that European people want change."
"The EPP is the party which has run Europe for the last 10 years during the economic crisis and they were the big losers even though they remain the biggest group in parliament" Sergei Stanishev
Stanishev said the "big losers" in the election were the EPP, which he said had lost 60 seats and seen its share of the vote fall by some 20 per cent compared with the 2009 elections.
"The EPP is the party which has run Europe for the last 10 years during the economic crisis and they were the big losers even though they remain the biggest group in parliament."
He said the Socialist vote share had remained stable compared with five years ago but voiced veiled disappointment that it had not done better. Even so, he said he was confident the party remained well placed to achieve its objectives in the next legislature, including further regulation of financial markets.
He also praised his colleague, German MEP Martin Schulz, a candidate for the commission presidency and parliament's president, for an "outstanding" electoral campaign, saying he had "reached" 150 million citizens via social media. "His profile is now even bigger than it was before the election."
Stanishev. who has led the Bulgarian Socialist party since 2001, also insisted that member states must "take account" of the outcome of the vote in deciding the next commission head, adding that, on this, he believes PES are in a "stronger position" than the EPP.
Addressing the same conference, PES general secretary Achim Post said, "It is now up to the political group leaders to form a 'stable' majority and the Socialists will play a decisive role in this."