A massive air, land and sea search was launched for a privately owned helicopter carrying five businessmen bound for Dublin which vanished off the Welsh coast.
The UK Coast Guard is now co-ordinating the search operation for the Airbus/Eurocopter Twin Squirrel red helicopter which vanished off radar in the Caernarfon Bay area.
Contact with the aircraft was lost shortly after 4pm after the helicopter left an airfield at Milton Keynes in the UK bound for Dublin.
The aircraft was due to land at Weston Airport in Lucan, Co Dublin.
The aircraft was last detected in the Caernarfon Bay area, less than 20km off the Welsh coast.
In a statement, the UK Coast Guard confirmed that its air, sea and land-based assets were all involved in the extensive search.
Two UK Coast Guard helicopters were sweeping areas off the Welsh coast last night, using infra-red camera technology to search for signs of debris.
It is understood that no mayday or distress call was received from the helicopter, whose pilot was said to be very experienced.
"The UK Coast Guard search and rescue helicopter based at Caernarfon has carried out an extensive land-based search of fields and airfields within the Caernarfon area and the helicopter based at St Athan has joined the search this evening," a spokesman for the Coast Guard said last night.
"A mayday relay broadcast has been issued to all vessels passing through the Irish Sea route to contact the Coast Guard if they have seen anything. A search of the Irish Sea route is commencing this evening."
North Wales Police have also been notified of the search operation.
"We have established a search plan between Caernarfon Bay and the shore of Dublin and currently have two Coast Guard helicopters searching the route," UK Coast Guard duty commander Peter Davies confirmed.
"We are in the process of contacting all the vessels in the Irish Sea that were in the vicinity around midday and we are continuing mayday broadcast action for any vessels currently in the area to contact us if they have further information."
"At the moment we are appealing to these vessels and to the public that if they have seen a twin squirrel red helicopter to please contact 999 and ask for the Coast Guard."
The twin squirrel helicopter is one of the most popular light types operated by private firms.
It has an excellent safety record.
Last night, search operations were hampered by poor weather conditions and reduced visibility.
Heavy rainfall is set to continue today which could hinder search operations.
This incident comes just a fortnight after the Irish Coast Guard Rescue 116 helicopter crashed off the north Mayo coast.
Captain Dara Fitzpatrick (45) and Captain Mark Duffy (51) died in the tragedy.
The funeral of Mark Duffy is due to take place this afternoon in his hometown of Dundalk in Co Louth.
Coastline searches for winch operator Paul Ormsby (53) and Ciaran Smith (38) have been extended after a piece of wreckage was found at the weekend.
Last night, many Welsh politicians expressed their thoughts and concerns for those on board the helicopter.