The European Parliament has refused to meet a delegation of Pakistani MPs that included an Islamic scholar it said had close links to the Taleban.
Mr ul-Haq (L) is part of the hardline Islamist alliance in Pakistan
It said it could not meet an individual who did not meet its "ideals of democracy, equality and human rights".
Senator Maulana Sami ul-Haq also heads a seminary in Pakistan that has been called the "University of Jihad".
Pakistan's ambassador in Brussels said the parliament's move was "unfortunate" and an "opportunity missed".
The Pakistani delegation of eight senators - four from the government and four from the opposition, including Mr ul-Haq, are heading home.
British Labour MEP, Neena Gill, leader of the EU parliament's South Asia inter-parliamentary committee, said: "The European Parliament espouses the ideals of democracy, equality and human rights.
"While we have members who represent all shades of the political spectrum, we are all working within the framework of a fully functioning democracy.
"We cannot condone therefore individuals who place themselves outside these parameters, for they represent everything we stand against."
Ms Gill said she would recommend improving the vetting system so that the parliament was more aware of the composition of delegation members.
The Pakistani delegation was due to meet the parliament's South Asia committee, the chairman of the Foreign Affairs Committee and a range of other MEPs.
Pakistan's ambassador in Brussels, Saeed Khalid, expressed disappointment at the cancellation.
"Even if you grant that you have a big difference of opinion you should talk about it. And this was an opportunity missed," he told BBC News.
Mr ul-Haq is part of the hardline Islamist alliance that forms a strong bloc in Pakistan's lower house.
The BBC's Haroon Rashid visited his seminary in Pakistan's North-West Frontier Province in 2003.
Graduation at Mr ul-Haq's 'university of jihad' in Pakistan
Our correspondent witnessed the graduation of 600 students pledged to fighting a jihad, or holy war, against enemies of their religion.
The convocation was full of slogans in support of Afghanistan's ousted Taleban regime, al-Qaeda's leader Osama Bin Laden and jihad, he reported.
Mr ul-Haq's advice for the Pakistani and Afghan students was to wage holy war until the "evil force" is defeated.
"Being watchmen of your religion, you are naturally the first target of your enemies," he said.
He had previously sent a batch of 2,000 Afghan students back to their homeland to aid the then ruling Taleban in its fight against the warlords of the Northern Alliance.