Jan Royall, a former aide to Neil Kinnock, has begun her job as the head of the European Union's office in Wales with calls for her to stand down.
Romano Prodi has been called on to investigate the appointment
BBC Wales' Dragon's Eye programme revealed that Hans-Gert Poettering, leader of the European Christian Democrat and Conservative group, is to raise the matter with Romano Prodi.
He will tell the president of the EU Commission, the move was "an inappropriate political appointment."
This call is echoed by Plaid Cymru and Conservative leaders in the Welsh assembly.
Plaid Cymru's deputy group leader Rhodri Glyn Thomas and Conservative leader Nick Bourne believe that Ms Royall's position is untenable because of her Labour connections.
Ms Royall was formerly a special adviser to Mr Kinnock and sought selection as Labour's candidate for Ogmore in the 2002 by-election.
The job as head of the EU's office in Wales, which carries a maximum salary of £89,000 year, was not advertised externally and was confined to EU civil servants.
Ms Royall has been silent in the face of criticisms
The head of the EU's offices in the UK, Jim Dougal, told BBC Wales that the appointment was above board and that proper procedures were followed at every stage.
And Neil Kinnock, Vice President of the European Commission, told the Dragon's Eye programme: "I was not involved in the appointment in any way since such procedures and posts are not in my portfolio.
"I understand that there were other applicants for the post and, in accordance with all the proper rules, Jan Royall was appointed on merit."
The appointment of Ms Royall was also defended by Eluned Morgan, one of Labour's two Welsh MEPs.
She said that Ms Royall had been appointed on merit and maintained that an individual's affiliation to a political party should not be reason for excluding them from applying for jobs.
I was not involved in the appointment in any way
The two main opposition parties in the assembly, however, say they are concerned that Ms Royall's high profile connections with leading Labour politicians, including the Kinnocks, will be a barrier to impartiality in the job.
They are particularly concerned that this will be a factor in the run up to next year's European elections and have repeated their demand that Ms Royall steps down immediately.
Ms Royall, who will be expected to represent the EU in Wales and Wales in Brussels, has so far refused to respond to the demands for her resignation and has turned down all requests for interviews.