He was speaking to reporters after meeting May in London on Thursday, the first one-on-one meeting between the pair.
The rights of Europeans in the UK was one of the red lines contained in a resolution recently adopted by Parliament which defines the guidelines for the upcoming Brexit negotiations.
After the meeting, Tajani said, "I underlined during the meeting that for the European Parliament this is most important point. We must strongly defend the right of EU citizens living in the UK and have a clear framework in the next months."
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He added, "For us it's important to ensure that Brexit does not have negative effects on their life and rights they are enjoying."
Tajani, who later also met NGOs to discuss Brexit-related issues, described the protection of rights for EU citizens settled in the UK and Britons living in rest of Europe as a red line in negotiations.
"This is very clear in the text approved by a very large majority by the European Parliament. For us, it is a priority and it is a red line. It's impossible to go back."
Tajani said he had so far failed to secure any firm commitment from May to address Parliament, something MEPs had been calling for.
On the possibility of progressing trade talks with Britain, he said, "We hope before the end of 2017 to have a framework on the exit and immediately afterwards it is possible to start with the negotiations.
"I will inform the European Council of this meeting and that it was positive. It's the beginning, there's a long way but it's important to start well."