The privately-owned twin squirrel red helicopter is believed to have left Milton Keynes to fly to Dublin, via Caernarfon Bay, before disappearing on Wednesday afternoon.
An extensive search of the Irish Sea and Snowdonia was launched at around 4.15pm after the Distress and Diversion system lost radar contact with the helicopter.
Two UK Coastguard search and rescue helicopters based at Caernarfon and St Athan were scrambled to scan the area, with all vessels on the Irish Sea route at the time also asked to report sightings.
The coastguard later said the overnight search had been scaled back due to poor conditions, although rescue teams were continuing to scour the mountain range.
A spokesman said: “Due to low-lying cloud, visibility has been extremely poor and has hampered the search efforts.
“The helicopters have now returned to their respective bases and there are no plans at this stage to continue the aerial search unless the weather improves significantly.
“North Wales Police have taken over the search operation and have called in mountain rescue teams to continue the land-based search in the Snowdonia area.”
The Eurocopter AS55 Ecureuil 2 (Twin Squirrel), now made by Airbus, is a widely used twin-engine light utility helicopter which has a reputation for durability and requiring low maintenance.
First introduced in the mid-1970s, the all-weather aircraft has evolved through several variants and is used by both civil and military applications.
With a cruising speed of 140mph and a range of nearly 440 miles, the model is used for extended low-level flights involved in filming and surveying.
Champion rally driver Colin McCrae was at the controls of a Twin Squirrel with his five-year-old son Johnny and his friend Ben Porcelli, six, on board in 2007 when it crashed in the grounds of his Lanarkshire home, killing all three.