The Council for National Parks will try to appeal against a High Court ruling that cleared the way for the Bluestone holiday village in Pembrokeshire.
The CNP's legal challenge was resfused last month
Last month Mr Justice Jack found there was nothing unlawful and no bias in the granting of planning permission by Pembrokeshire Coast National Park.
The £60m Bluestone village includes 335 timber lodges, a water attraction, sports club, snow dome and beauty spa.
The CNP claims the national park broke its own rules in backing the scheme.
Ruth Champers, the watchdogs head of policy, said: "CNP is a small charity with limited resources but we have decided to fight on.
"We do not dispute that Bluestone would bring some economic benefit to the area but our view remains that the development does not have to be located in the national park for it to proceed."
Pembrokeshire Coast National Park Authority said it was disappointed the CNP had decided to continue its fight.
Chairman Stephen Watkins said: "The National Park will robustly oppose the granting of leave to appeal and has already made an initial submission to the judge.
"We very much regret the further period of uncertainty
which has again been introduced."
Backers of the project say it will create 800 jobs in an unemployment black spot and inject £32m a year into Pembrokeshire's economy.