A number of US states had argued the plastic guns are "untraceable and virtually undetectable"
A judge in the US has blocked the release of blueprints for 3D printed plastic guns.
The plans for the weapons were due to be available to download from today, after activists reached an agreement with the US government earlier this year.
It marked a reversal from the federal government's past opposition to the release of the files, with officials having previously claimed the release would violate firearm export laws.
Following the recent settlement, a group called Defense Distributed - which says it is "dedicated to the advancement of American gunsmithing and the expansion of the Second Amendment" - had been planning to release the files online today, August 1st.
The Washington Post reports that a number of files had already been uploaded last Friday, with blueprints for plastic semiautomatic rifles downloaded around 1,000 times.
However, a number of US states went to court - raising concerns the weapons would be untraceable and would pose a 'serious threat to national security and to public safety'.
Eight states and the District of Columbia filed a lawsuit against the Trump administration to block the release of the files.
In a ruling granting a temporary injunction, Washington state district judge Robert S Lasnik said: "Plaintiffs have shown a likelihood of irreparable injury if the downloadable [Computer Aided Design] files are posted tomorrow as promised.
"If an injunction is not issued and the status quo alters at midnight tonight, the proliferation of these firearms will have many of the negative impacts on a state level that the federal government once feared on the international stage".
A further hearing on the issue will now take place on August 10th.
Washington state's attorney general Bob Ferguson's celebrated the ruling.
He observed: "[It's a] complete, total victory. We were asking for a nationwide, temporary restraining order putting a halt to this outrageous decision by the federal government to allow these 3D, downloadable guns to be available.
"These ghost guns are untraceable, virtually undetectable and, without today’s victory, available to any felon, domestic abuser or terrorist."
In a brief tweet on Monday, Donald Trump said he was 'looking into' the sale of 3D-printed guns, saying he had spoken to the National Rifle Association (NRA) about them:
I am looking into 3-D Plastic Guns being sold to the public. Already spoke to NRA, doesn’t seem to make much sense!— Donald J. Trump (@realDonaldTrump) July 31, 2018