A bundle of party balloons caused a business jet's flight crew to shout "watch out!" and take avoiding action shortly after take-off from London City Airport.
The Cessna 560 reached an altitude of 2,300 feet when the near miss occurred, the UK Airprox Board (UKAB) said.
One of the pilots on board spotted a group of 20 black and yellow balloons up to three metres wide approaching the aircraft and shouted "watch out!" to their colleague.
Auto-pilot was then disconnected by the crew, who manoeuvred the jet to miss the balloons.
The bundle passed two metres above the plane and 10 metres to its left hand side.
The UKAB categorised the October 6 near miss as having the most serious risk of a collision.
"Providence had played a major part in the incident and/or a definite risk of collision had existed," the board concluded.
It was one of 15 near misses involving aircraft which were discussed at the UKAB's latest monthly meeting.