A £20,000 reward is being offered in connection with the armed raid on Leyland's Bent Lane sub post office.
The reward has been put up by Post Office Counters and the Federation of Sub Postmasters and will be paid out on the arrest and conviction of the gang.
The masked robbers - armed with a screwdriver and a knife - broke into the premises just after midnight on Friday, August 20. Sub postmaster Roy Farron tackled one of the men on the stairs, believing him to be on his own.
He was then beaten by the gang, dragged around and tied up along with his wife.
The thugs then threatened to kill the couple before stealing £1,000 and cigarettes.
They drove off in a blue Ford Sapphire Cosworth with false numberplates, stolen from Shotton, Deeside.
Two of the thieves were seen driving at high speed towards the M6 by a motorway patrol car. After a brief chase, the Cosworth was lost after it left the M6 at Shevington and the gang tried to steal a Toyota Carina before taking a Ford Fiesta from Skull House Lane - found dumped the following day in Butchers Lane, Aughton.
DCI Mike Barton, leading enquiries into the raid, said: "This was a terrifying attack with gratuitous threats of violence which left both the sub postmaster and his wife severely shaken.
"The fact the reward is so big reflects just how seriously we are treating this incident.
"We are keen to speak to anyone who saw suspicious activity in Leyland, Appley Bridge and Aughton and are still unsure how many vehicles may have been involved."
DS Dave McKenna added: "We believe there was some element of planning prior to the offence.
"We are still speaking to some witnesses and would like more people to come forward."
Merseyside police are following leads up in that area following reports that the men spoke with Liverpudlian accents.
Anyone with information is asked to contact Leyland police on (01772) 433561 or Crimestoppers on 0800 555111.
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