President Vaclav Klaus said he would invite the Civic Democrats to form a new government on Monday, according to the Czech constitution.
Incomplete results suggested Topolanek's party had won 35.4 per cent of the vote, ahead of Prime Minister Paroubek's Social Democrats on 32.3 per cent.
Mr Paroubek is refusing to concede and has warned he may mount a legal challenge.
He says leftist parties will still end up with the majority of the vote and accused the right of dirty tricks during the election campaign.