She raised concern that such a deal could put the Northern Ireland peace process at risk as the Government would no longer be an "honest broker" in the Good Friday Agreement.
"I say that not just because of some of the views of the DUP that, perhaps not all of us, but many of us feel deeply uncomfortable about, but I also say that because of a real concern about the disregard that is being shown for the Northern Irish peace process".
The remarks come amid mounting criticism that Mrs May's proposed deal with the Unionist MPs - needed for the Tories after they lost their majority - jeopardises peace in Northern Ireland.
"A little side bargain to keep Theresa May in power, a temporary little arrangement won't have any integrity and certainly is not as important as the integrity and the needs of the people who live in Ireland".
Sinn Féin, he said, "won't interfere in British affairs".
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"We are consistently making the case that our institutions are under resourced, have been undermined by austerity and need to be properly funded".
May's office said Northern Ireland's five main parties would take part, but a spokesman for the DUP did not immediately respond to a request to confirm their participation.
Although elections for the assembly took place in a March, it can only meet if the two leading parties, the Democratic Unionist Party (DUP) and the pro-republican Sinn Fein, agree to work together by appointing a first minister and deputy first minister.
"The prime minister will have to do a lot more to convince us that the DUP tail isn´t wagging the Tory dog", Colum Eastwood, leader of the nationalist Social Democratic and Labour Party (SDLP), said in a statement.
Speaking in London on Wednesday, a senior Conservative official said talks with the DUP are progressing well and there is broad agreement on the principles of the June 21 Queen's Speech, in which the monarch sets out the government's headline plans.