(Updates with upper house of parliament's vote)

WARSAW, May 14 (Reuters) - Changes to a Polish law that would result in the dismissal of the head of the country's telecoms regulator could infringe on EU law, the European Commission said in a letter to the Poland's digital minister, adding to already strained relations.

In power since 2015, the conservative Law and Justice (PiS) party has been in increasing conflict with Brussels over issues including judicial reforms, refugees, and climate change.

In a recent anti-crisis package Poland proposed changes that would result in dismissal of the telecoms regulator's head and take the right to approve a candidate for the post from the upper house of parliament, where the opposition holds a majority.

Why would these "... changes need to be introduced with an urgency legislation devoted to addressing the current COVID crisis, despite not being related to the COVID crisis," commissioner for the Internal Market, Thierry Breton, wrote in a letter seen by Reuters on Thursday.

"The Court of Justice of the European Union already ruled that the dismissal of the President of a national regulatory authority, before the expiry of their terms of office, infringes EU law, in the absence of any rules guaranteeing that such dismissals do not jeopardise its independence and impartiality," he added.

Critics say that PiS wants to dismiss the regulator's head as he is considered market-oriented and uncontrollable.

The changes were rejected on Thursday by the upper house of parliament, so the bill will now come back to the lower house of parliament, where the ruling party holds a majority and has the power to decide the ultimate fate of the legislation.

Poland's Digital Ministry told Reuters that changes are aimed at implementing requirements of the European Electronic Communications Code and in line with EU regulations.

"The government and parliament are currently trying to minimize the number of separate legislative initiatives in order to limit the gathering of members of parliament," it said explaining why changes are being introduced with an urgency legislation.

The EC was not immediately available for a comment.

(Reporting by Anna Koper; Editing by Steve Orlofsky)