Instead, he will go ahead with his plans to travel to the United States to watch his son, Max, play baseball for New Zealand in a major tournament.
Mr Key said he had spoken to the families of Lance Corporals Pralli Durrer and Rory Malone to explain.
"In the end it's a very, very difficult decision. I've got to let somebody down, but my son makes huge sacrifices for me and my job and, in the final analysis, I've just decided it's probably the right thing to do - to go and support him."
Max Key is in the New Zealand under-17s All Stars baseball team, the first side from New Zealand to qualify for a world series. The Senior League World Series starts tomorrow in Bangor, Maine.
The funerals of the men killed last Saturday when going to the aid of Afghan police in Bamiyan are expected to be tomorrow in Christchurch.
Deputy Prime Minister Bill English will attend the ceremony in Mr Key's place.
Labour Party leader David Shearer will also attend. He would not comment on the Prime Minister's decision, saying it was Mr Key's to make.
It will be the first time Mr Key has not attended the funerals of defence personnel killed on active service since becoming Prime Minister in 2008.
In 2010, he abandoned a proposed trade mission to the Middle East to return to New Zealand for the funerals of Flying Officer Daniel Gregory, Corporal Benjamin Carson and Flight Lieutenant Hayden Madsen, who died when their Iroquois crashed on Anzac Day. Mr Key flew home from Gallipoli for the funerals, then left again the next day on a secret trip to Afghanistan.
He was criticised at the time by Cognition Education chief executive John Langley for abandoning his international obligations by leaving a delegation of 90 business people he was taking to Saudi Arabia, Bahrain, Kuwait and the United Arab Emirates. Trade Minister Tim Groser stayed on in his place.
The bodies of Lance Corporals Durrer and Malone, both 26, were farewelled with a haka and ramp ceremony at Bagram Air Base in Afghanistan on Monday.
Their coffins were escorted by fellow members of the New Zealand Provincial Reconstruction Team on a US Air Force C-17 and another ramp ceremony was held when they arrived at Christchurch last night.
Six other New Zealanders injured in the same skirmish with insurgents are in Germany recovering and are not expected to be home in time for the funerals.
- Claire Trevett of the New Zealand Herald
