Ewa Kopacz made the disclosure during a joint statement with Harper in Warsaw, catching officials with the Prime Minister's Office off guard and raising questions about whether operational safety may have been compromised.
"You might want to know that the prime minister is going to Gdansk tomorrow... where he will be visiting Canadian frigates participating in the military exercises," Kopacz said through a translator to a room packed with journalists.
The trip had not been publicly announced by Harper's office. The leaders did not take questions from journalists, making joint statements instead.
Harper did not appear to be thrown off stride by the revelation.
Defence Minister Jason Kenney later confirmed that the prime minister would indeed be visiting HMCS Fredericton as a show of solidarity with Canadian military personnel and their allies.
"I'm not going to discuss exactly what we might be doing in the next 24 hours except to say that obviously our prime minister will be visiting one of the frigates that's been on joint training exercises in the Baltic Sea with a large-scale NATO naval exercise," Kenney said.
Kenney was in Warsaw for meetings with Polish military officials.
Earlier, Harper took part in a wreath-laying ceremony at the Tomb of the Unknown Soldier, in the historic square where Polish-born Pope John Paul II delivered a stirring homily in June 1979 that stressed human dignity in support of the Poland's anti-communist Solidarity movement.
He also met with Kopacz and Polish President Bronislaw Komorowski.
Poland is a major European ally of Canada, which deployed more than 200 troops here on NATO training exercises last year.
Previously, Harper attended the G7 summit in Germany, where the continuing unrest between Ukraine and Russia was a hot topic.
"Canada and Poland served side by side in Afghanistan, and we now stand together in promoting freedom and democracy in Ukraine," the prime minister said in his address.
Harper enjoys strong relations with Poland after forging a bond with Kopacz's predecessor Donald Tusk, who became European Council president last year.
The Polish diaspora in Canada is estimated to be about one million people, which Harper said represents the strong bond between the two countries.
Before the G7, Harper stopped in Ukraine to pledge Canadian solidarity in the country's fight against Russian-backed rebels in the country's east. While he was there, months of shaky ceasefire gave way to some of the worst violence of the conflict.
Standing next to Kopacz, Harper reiterated Canada's solidarity in standing with its allies against Russian President Vladimir Putin in east Ukraine. Harper's trip to the frigate was part of that effort, Kenney said.
"One of the reasons the prime minister is here is to underscore solidarity with our allies in eastern Europe — both Ukraine, Poland and the three Baltic states — who are concerned obviously about Vladimir Putin's policy of aggression."
Kopacz's gaffe came on a morning when she awoke to major development in a simmering domestic political scandal that has been dogging her government. Like Harper, she will have to fight a national election this coming October.
Her remark also overshadowed Harper's announcement for the day — the signing of a defence co-operation agreement between Poland and Canada.
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