BNP leader Nick Griffin, who has said global warming is “essentially a hoax”, will be at the Copenhagen climate change conference.
The MEP will be there representing the European Parliament, as he sits on its environment committee.
The BNP said he would be “the only politician there prepared to say that the science is somewhat dodgy”.
Green Party leader Caroline Lucas told the BBC any suggestion he would have any real influence was “a myth”.
Delegations from 192 countries will hold two weeks of talks in Copenhagen in December aimed at establishing a new global treaty on climate change.
In a speech to the European Parliament last week Mr Griffin claimed those who warned of climate change were “anti-western intellectual cranks” and described climate change as “a secular religious hysteria”.
A BNP spokesman said his appearance in Copenhagen would be a “big opportunity” for the party because “people assume we are only a one-trick pony only interested in race and immigration”.
But Ms Lucas, who is also an MEP, told BBC One’s Andrew Marr Show: “He is one of a number of members of the European Parliament who will go on a delegation.