Ann Linde told SVT’s Agenda said the PM’s failure to secure a majority government after last week’s General Election had left Britain in a weakened position.
The cabinet minister added the result of the snap election was not only bad for the UK, but also the EU as it could delay Brexit negotiations.
Ms Linde said: “The UK election result is not good for EU or Sweden.
“We might not be able to start negotiations as planned. It’s not a good situation.”
Ms Linde said negotiations are about giving and taking, and Mrs May would have to compromise with the bloc if she wanted to secure a Brexit deal.
She said: “In negotiations, you have to give and take. There is pressure from the media in Great Britain to win in every part of the negotiations.
“May has to reach compromises and this will be much harder now that she doesn’t have a majority.
“The fact of the matter is, now strong demands will be set out both from within her own party and from the other parties, against such a ‘hard’ Brexit.
“’Hard’ Brexit means leaving both the single market and customs union. Now more people are setting out demands for remaining in the customs union or the single market.”
Ms Linde added: “The problem is, if the UK wants to remain in the inner market they will have to concede on the most important demands, they don’t want to pay, they do not want European Courts to have jurisdiction, and they don’t want freedom of movement.
“That isn’t possible if you want to be part of the single market.”
On Tuesday, the EU’s Brexit negotiator Guy Verhofstadt claimed he spoke on the behalf “ordinary citizens” in Europe as he demanded the UK hurry up and start divorce talks.
Mr Verhofstadt said he was growing impatient with Britain, as the decision to leave the bloc was creating “a lot of trouble”.
Speaking to Sky News, Mr Verhofstadt said: “I don’t know if it is the responsibility of the UK Government. Whether the UK Government stick to the position the ‘hard’ Brexit described in the letter of March 29 or will they adapt that position because of the election, that is the responsibility of the UK Government.
“What I want, and what we want in Europe, is that we can stall this, the whole debate about the Brexit started three years ago, one year ago there was the result of the referendum, three months ago was the letter of March 29, starting Article 50 procedure, we are still not negotiating.
“That cannot continue because the uncertainty for the citizens and the businesses has to stop as fast as possible.
“My impatience is mainly driven by the uncertainty that creates a lot of trouble for EU citizens, for UK citizens living on the Continent and for the businesses working on the Continent and in Britain.
“That’s the problem, my impatience is only the translation of the impatience of the ordinary citizens and businesses.”