Brian Fletcher travelled from his home in Bury on Saturday, March 22 to Tockholes and has not been seen since.
Emergency services have carried out searches using underwater search teams, specialist dogs, police drone and helicopters and mountain rescue volunteers.
Today, Monday, Lancashire Police said that the force is now standing down the search but will continue to investigate any new leads or information which they receive.
Brian was wearing a tan-coloured cap, blue jumper and cargo trousers on the day he went missing and his car was found soon afterwards parked by on the A675, near to Tockholes Road.
A Lancashire Police spokesperson said: “It is more than three weeks since Brian Fletcher went missing in the Tockholes area of Lancashire.
“Brian, 80, travelled from his home in Bury on Saturday, March 22 to Tockholes, an area he visited regularly as he’s a keen walker and bird watcher.
“His car was found parked in a layby on the A675, near to Tockholes Road.
“We have carried out extensive searches for Brian since he was reported missing, the search covering approximately 36 square kilometres of moorland, woodland, watercourses and reservoirs.
“Our search teams have been supported by local officers, specialist search dogs, the police helicopter and police plane, the police drone team, the North West police underwater search team, and our mounted branch.
“Our colleagues at Greater Manchester Police have assisted with their search dogs.
“Aiding us have been Bolton Mountain Rescue, Bowland and Pennine Mountain Rescue, Rossendale Mountain Rescue, Holme Valley Mountain Rescue, Calder Valley Mountain Rescue and Oldham Mountain Rescue.
“Lancashire Fire and Rescue Service have assisted with their service dogs, and the Coastguard helicopter was used.
“We are very grateful for the support and expertise which those organisations have provided in difficult terrain and conditions. And of course to members of the public who have also searched for Brian whilst walking in the Tockholes area.
“Dashcam and CCTV footage from the area has been viewed extensively during the search for Brian.
“Whilst we have now exhausted all search enquiries and so for the moment the searches have been stood down, we will of course continue to act on any new information reported to us.
“Brian’s family have been made aware of this and are fully understanding of the position.
“The family have expressed their appreciation of the extensive searches carried out by police and all the agencies involved, and to the members of the public who kindly offered their help and assistance in looking for Brian.
“We and the other organisations involved in the search may conduct re-searches in the area to account for changes in environmental conditions or if new information comes to light.
“If you are in the Tockholes area and see anything which you think might help find Brian, or you were there on March 22 and remember seeing someone match his description, please contact police.
“When Brian went missing, we believe he was wearing a tan-coloured cap, blue jumper and cargo trousers – as seen in the second image which was taken before his visit to Tockholes.
“Anyone with information, please contact 101 – quoting log 1,544 of March 22.”
