"This is a dangerous move and certainly China will oppose it," China's UN ambassador Wang Guangya said at the UN headquarters in New York on Thursday.
"It will split the house and destroy the unity and also derail the whole process of discussion on big UN reforms."
China has opposed Japan being granted permanent status on the Security Council, demanding it first correct its attitude to its wartime history.
Tensions between the two countries have risen in recent months.
Brazil, Germany, India and Japan have formed a group, called G4, to lobby for permanent seats on the Security Council.
It has circulated a draft resolution, which could be voted on at the UN General Assembly in September, proposing a 25-member Security Council, 10 more than now, with six new permanent members.
Wang said China leaned towards a rival plan, proposed by Italy, Mexico and Pakistan, to enlarge the Security Council to 25 members, but without additional veto-wielding permanent members.
"We see many good points in their formula because this will expand the Security Council and this will give certain members who they believe are important a longer term"
China's UN Ambassador
"We see many good points in their formula because this will expand the Security Council and this will give certain members who they believe are important a longer term," he said.
In the Italy-Mexico-Pakistan plan, some non-permanent members could be re-elected at the end of their two-year stints on the Security Council, unlike the current practice.
The G4 nations plan to put their motion to the General Assembly if they are certain they will get the support of two-thirds of the 191 UN members so that it will be passed.
The text does not say which countries should become permanent members but proposes two for Asia, two for Africa, one for Western Europe and one for Latin America.
Africa, Latin America, Asia and Eastern Europe would each get one of the new non-permanent seats.
India, Japan, Germany and Brazil say that all of the new permanent members should have the same right to veto a resolution as the current five permanent members: Britain, China, France, Russia and the US. But the US has opposed extending the veto.
China could not technically block a motion put to the General Assembly but could kill it off later.
The change to the Security Council would also require changes to the UN charter. This would have to be passed by the parliaments of two-thirds of the UN members, including the five permanent members.
Altering the charter is the fourth stage in the G4 plan. Wang said, "I hope it will not come to the fourth stage."
UN Secretary-General Kofi Annan also hopes to avoid a showdown.
Annan is hoping Security Council
reforms will run smoothly
"Ideally, consensus is what one should aim for, but if that were to fail and there is a broad agreement, one should be able to vote," he said.
Japan has made winning a permanent seat on the Security Council a top goal of its foreign policy. But China says Japan has not atoned enough for the past to deserve a seat.
China has strongly attacked Japan over Prime Minister Junichiro Koizumi's annual pilgrimage to a shrine that honours Japanese war dead, including 14 war criminals.
China has called Koizumi's visits to the Yasukuni shrine the biggest obstacle in bilateral relations.
Koizumi has defended his visits, saying the pilgrimage is a Japanese way to honour the dead. On Thursday, he again demanded that other countries not "interfere" and signalled he was ready to go again.