Match ends, Poland 1, Northern Ireland 0.
Poland 1-0 Northern Ireland
Northern Ireland's first match at a European Championship finals ended in defeat as Arkadiusz Milik gave Poland a 1-0 win in Nice.
Ajax star Milik slotted in after being picked out by Jakub Blaszczykowski early in the second half.
Poland had dominated the first half but were thwarted by some superb defending by Michael O'Neill's men.
Northern Ireland did not manage to get a decent effort on target and must now regroup to play Ukraine on Thursday.
Poland deserved the three points, the first time they have won an opening match at a Euro finals, inflicting a first defeat in 13 games on their Group C opponents.
Northern Ireland endured an anxious opening and must have been relieved to get to the break still on level terms - they have now conceded just one first-half goal in their last 21 matches.
But the mood changed when Milik, scorer of 24 for his club last season, took one touch before finding the net with a left-foot shot.
Northern Ireland's best hope was a well-worked free-kick which ended with skipper Steven Davis just failing to get in his shot.
First night nerves?
More than 10,000 green-shirted fans travelled to Nice full of an optimism fired by their team's qualification and 12-match unbeaten run.
But that fantastic vocal support failed to inspire Northern Ireland, who struggled badly in the opening exchanges, letting Poland dominate.
And despite the significance of the match for O'Neill's side, it will not be one they will look back on with much pleasure.
On a positive note, they finished the match much better than they had started it, with substitute Washington providing support for sole striker Lafferty, while they also produced a determined defensive display to shackle Robert Lewandowski, one of Europe's leading strikers.
Setback for manager O'Neill
O'Neill looked crestfallen at the final whistle as his side failed to take any points from their first game.
But his team had not managed to keep enough possession and did not create sufficient chances.
O'Neill decided not to go with the expected 3-5-2 formation, and started with five at the back, switching to four for the second half.
Now he must rally his tired players to take on Ukraine in their next match in Lyon on Thursday.
Realistically, Northern Ireland must win that one or face the prospect of having to beat world champions Germany in their final Group C fixture to have a chance of qualifying for the last 16.
That will mean a change of approach, and not letting their opponents dictate matters.
Man of the match - Bartosz Kapustka
What they said
Northern Ireland captain Steven Davis, speaking to BBC Sport: "It's a disappointing evening. We have to braver on the ball but it's a real learning curve for us. We always knew it would be difficult but we weren't good enough."
Northern Ireland striker Kyle Lafferty, speaking to BBC Sport: "I think we were overawed. That was not the team that has got us here, we have lacked a lot of that today and have come away with no points.
"We knew it would be tough, but I think the whole performance was not a Northern Ireland performance and on Thursday we will go and put it right."
The stats you need to know...
- Poland recorded their first win at the European Championship finals, after failing to win their previous six matches (D3 L3).
- Northern Ireland have failed to win their last six matches at major tournaments (D2 L4).
- Poland also recorded their first clean sheet in the competition.
- Northern Ireland registered just two attempts at goal - the joint-lowest total by a team in a European Championships match since the finals expanded from four to eight teams in 1980.
- Poland completed more passes against Northern Ireland than in any other previous Euros match (431).
- Arkadiusz Milik (at 22 years 104 days) is the youngest goalscorer for Poland in European Championship finals history.
Northern Ireland need a positive result when they face Ukraine in Lyon on Thursday (17:00 BST) while Poland take on Germany on the same day in Paris (20:00 BST).
- 20PiszczekBooked at 89mins
- 16BlaszczykowskiSubstituted forGrosickiat 80'minutes
- 5MaczynskiSubstituted forJodlowiecat 78'minutes
- 21KapustkaBooked at 65minsSubstituted forPeszkoat 88'minutes
- 20CathcartBooked at 69mins
- 17McNairSubstituted forDallasat 45'minutes
- 6BairdSubstituted forWardat 76'minutes
- 3FergusonSubstituted forWashingtonat 66'minutes
- Ovidiu Hategan
- Shots on Target
Second Half ends, Poland 1, Northern Ireland 0.
Foul by Robert Lewandowski (Poland).
Jonny Evans (Northern Ireland) wins a free kick in the defensive half.
Arkadiusz Milik (Poland) wins a free kick in the attacking half.
Foul by Oliver Norwood (Northern Ireland).
Delay over. They are ready to continue.
Delay in match Wojciech Szczesny (Poland) because of an injury.
Offside, Northern Ireland. Oliver Norwood tries a through ball, but Kyle Lafferty is caught offside.
Lukasz Piszczek (Poland) is shown the yellow card for a bad foul.
Foul by Lukasz Piszczek (Poland).
Stuart Dallas (Northern Ireland) wins a free kick on the left wing.
Substitution, Poland. Slawomir Peszko replaces Bartosz Kapustka.
Attempt missed. Grzegorz Krychowiak (Poland) right footed shot from outside the box is close, but misses to the right. Assisted by Tomasz Jodlowiec.
Foul by Robert Lewandowski (Poland).
Oliver Norwood (Northern Ireland) wins a free kick in the attacking half.
Attempt saved. Arkadiusz Milik (Poland) right footed shot from the centre of the box is saved in the top centre of the goal.
Corner, Poland. Conceded by Jonny Evans.
Attempt blocked. Kamil Grosicki (Poland) right footed shot from the right side of the box is blocked. Assisted by Robert Lewandowski.
Substitution, Poland. Kamil Grosicki replaces Jakub Blaszczykowski.