The UK's decision is contrary to its obligations under Article VI of the Nuclear Non-Proliferation Treaty (NPT), Xinhua news agency quoted Stephane Dujarric, spokesman for UN Secretary-General Antonio Guterres, as saying at a briefing on Wednesday.
It could have a damaging impact on global stability and efforts to pursue a world free of nuclear weapons, he said.
"At a time when nuclear weapon risks are higher than they have been since the Cold War, investments in disarmament and arms control is the best way to strengthen the stability and reduce nuclear danger," Dujarric was quoted as further saying.
The UK government on Tuesday announced its plan to increase the number of nuclear warheads to not more than 260, reversing its previous policy of reducing its overall nuclear warhead stockpile ceiling to not more than 180 warheads by the mid-2020s.
Outlining the strategy to MPs, Prime Minister Boris Johnson said the UK would have to "re-learn the art" of competing against countries with "opposing values".
But he added the UK would remain "unswervingly committed" to the NATO defence alliance and preserving peace and security in Europe.
Speaking to the BBC on Tuesday, Beatrice Fihn, head of the International Campaign to Abolish Nuclear Weapons, described the UK's decision to change its nuclear provision as "outrageous, irresponsible and very dangerous".
She said it went against international law and did not address the real security threats faced by the UK such as climate change and disinformation.