The decision was taken by Guthrie's three grown-up children after their mother Sandra told them before her death last October that she wanted her late husband's GC to be sold. She had always been concerned for its security and had kept it hidden in a "safe place" at her home, the Herald on Sunday has been told.
In a rare move, Manatū Taonga Ministry for Culture and Heritage approved an application for its export.
"As with other applications, the ministry followed the procedures under the Protected Objects Act 1975, including seeking advice from expert examiners," a spokeswoman said.
Ashcroft, who owns the world's largest collection of Victoria Crosses and more than 20 George Crosses, has vowed to display the Guthrie GC publicly at the world-renowned Imperial War Museum in London later this year.
"I feel proud and privileged to have become the custodian of this iconic George Cross that was awarded posthumously to such a courageous police officer," the businessman, philanthropist, political pollster, and Belize-resident told the Herald.
"I am hugely grateful to Stewart and Sandra Guthrie's grown-up children for the sensitive and discreet way that they have handled the sale of their father's decoration and for enabling his GC to go on public display.
"I want to assure everyone living in New Zealand that this is a much-treasured decoration that will be safe and secure in the Lord Ashcroft Gallery for a very long time to come."
The private deal was negotiated on behalf of the Guthrie family by Auckland medals dealer Aubrey Bairstow.
"The matter is private between all parties concerned but we feel that the outcome is the most respectful to the sacrifice the medals represent and we look forward to the medals being displayed in the collection of Lord Ashcroft housed at the Imperial War Museum, where a wider audience can appreciate the medals in context with other awards," Bairstow said in a statement.
Ashcroft is a keen supporter of New Zealand's military history after successfully putting up rewards for the safe return of 96 military medals, including nine Victoria Crosses stolen from Waiouru Army Museum more than a decade ago.
In 2015, he stepped in to prevent the last surviving Dambusters pilot, Squadron Leader Les Munro from selling his gallantry and service medals to raise money for the Bomber Command Memorial in London.
In return for Ashcroft donating $150,000 to the memorial appeal, Munro eventually gifted his medals to the Museum of Transport and Technology in Auckland. Munro died later that year, aged 96, content that his medals had remained in New Zealand.