Speaking at a conference at the Swedish-Finnish Cultural Centre in Helsinki, Wallström accused British leaders of having “badly handled” the Brexit process.
“I just think that they've made such a historical mistake and they've really created a problem for all of us,” Wallström said in remarks that were first reported by Politico.
“Our political project, the European Union, will suffer from this immensely and that has to be fully understood,” the Swedish foreign minister said.
“This is because of bad political leadership since a very long time in the UK I saw with all my years from the European Commission … there was nobody who would defend their [the UK’s, ed.] EU membership,” she added.
Wallström, who served in the European Commission from 1999 through 2010, made it clear that hard feelings are likely to remain when the UK formally leaves the EU on March 29th.
“I cannot forgive them for this,” she said.
Wallström's remarks came as the House of Commons prepared to debate amendments to PM Theresa May’s Brexit plan later on Tuesday.