The "Causeway cronyism" row has intensified, with criticism voiced from across the political spectrum.
The Belfast Telegraph revealed this week that party leader Ian Paisley had lobbied the Heritage Lottery Fund over a grant bid by prospective Causeway visitor centre developer Seymour Sweeney.
Dr Paisley incorrectly claimed that Mr Sweeney, who is a DUP member, had secured the support of heritage body Unesco for his project.
The DUP leader made the assertion in an angry letter to the HLF in January 2003, denouncing its decision not to financially back the developer's Causeway plans.
Unesco flatly contradicted his claim again yesterday.
MEP and ex-DUP figure Jim Allister last night said: "In light of today's clear declaration from Unesco that they never approved Sweeney's proposal and believe the visitors centre must be on the original site, the First Minister owes a detailed explanation as to how he, using the weight of his office as an MP, came to represent the opposite to the Lottery organisation."
Sinn Fein MLA Daithi McKay said his party would "keep up the pressure" on the Causeway dispute.
DUP Environment Minister Arlene Foster stated she was not aware of the developer's DUP affiliation when she said she was "of a mind" to approve his plans.
Dr Paisley's January 2003 letter to the Heritage Lottery Fund appears to have been signed on his behalf by his politician son Ian Paisley jnr.
Mr Paisley jnr claimed last night that the letter was being used for mischief-making.