Lancashire Police said its week-long amnesty will continue for another seven days.
Operation Sceptre was set to end today after being launched as part of a national campaign targeting knife crime and those who carry knives.
But the force tweeted on its accounts this morning: "We're extending our knife surrender for another week. Hand in your knives anonymously and without any repercussions."
It comes after officers visited businesses across Preston to carry out test purchases. It is illegal to sell blades to anyone under the age of 18.
Eight passed but one failed, police said.
Ch Insp Mark Baines said: "Too many families have been affected by knife crime with even one person being stabbed being one too many.
“The consequences of carrying and using a knife can be devastating and potentially life-threatening.
“I would appeal directly to people that if you know someone who is carrying a knife, don’t keep silent. Report it.
“The law is simple - it is illegal to carry a knife, even if it belongs to someone else.
“Anyone who is found carrying a knife and is intending to use it as a weapon – even in self-defence can be arrested, go to court and receive a police record or even a prison sentence of up to four years and an unlimited fine.”
Knife bins are being used at nine police stations in Blackpool, Burnley, Preston, Skelmersdale, Morecambe, Fleetwood, Blackburn, Chorley and Colne, during the surrender where people can hand in any knives.
They need to be safely wrapped in tape and newspaper before putting in the bins.
Ch Insp Baines added: “Lancashire Constabulary has a zero tolerance policy toward possession of knives in public and we take reports of knife crime very seriously.
“We will act on all intelligence received in a bid to prevent people losing loved ones and to detain those who intend to commit acts of violence using knives.
“If you have information about the possession of knives or offensive weapons or criminal activity relating to this type of crime, make a report online via the Lancashire Constabulary website.”
You can also anonymously report information on 0800 555 111.