Lawmakers in Poland, the largest country on NATO's eastern flank, last month backed the Security Action For Europe (SAFE) scheme, which would bring nearly 44 billion euros in loans.

The money is earmarked for air and missile defences, anti-drone technology and other equipment, as war rages in neighbouring Ukraine.

"I have decided not to sign the law that would allow Poland to take out the so-called 'Safe' loan. I will never sign a law that undermines our sovereignty, our independence, as well as our economic and military security," Nawrocki said in a televised address.

Nawrocki has been locked in a fiery row with the pro-European government of Prime Minister Donald Tusk, who denounced the veto.

"The president has missed the chance to behave like a patriot. A disgrace!" he wrote on X.

His spokesperson meanwhile denounced "an act of national treason".

In response to the growing threat from Russia, Poland has upped its defence budget to 4.8 percent of its GDP, one of the highest in NATO.

The government hailed the SAFE scheme as a big win for Poland, set to get nearly a third of the total 150-billion-euro pot.

Tusk has called an extraordinary cabinet meeting on Friday at 9:00 am (0800 GMT) in response to the veto.