Speaking in 30-minute long live interview on Swedish Radio's P1 channel, Bildt says the result of the referendum will be invalid, no matter which way people vote.
He also added that he sees no similarities with the situation in Kosovo, which broke out of Serbia and has since been recognised by Sweden, among others.
"There are important differences", Bildt said. "After a large war in 1999 Kosovo was placed under UN administration. A long political process preceded its independence, in which the EU, Serbia and Kosovo took part.
Crimea, on the other hand, has seen military intervention from Russia, tv stations have been closed, for example. This could have been an internal process within Ukraine, but that has not been the case. There is the risk of an annexation which has been forced by an invasion."
The foreign minister refused, yet again, to label the far-right Ukrainian party Svoboda, which has taken several posts in the new Ukraine government.
"I won't describe what that party is", Bildt told P1. "We accept the government which has been formed, with a lot of different people. It is a reasonable and democratic government and I don't want to play along with Russian propaganda."
Meanwhile, Bildt adds that EU sanctions aimed at specific Russian officials look like being introduced following Monday's meeting of EU Foreign Ministers. Unless something changes, which Bildt sees as unlikely.
"I find it hard to see that", Bildt says, "We had certain hopes, not exaggerated ones, for the meeting held yesterday between the American and Russian Foreign Ministers in London. It lasted for six hours, so the discussions were very extensive. But it was obvious towards the end that the Russian position hadn't changed at all."