Spain & Portugal bid to co-host World Cup
"We will create a joint commision to assess all FIFA conditions," Portuguese Federation president Gilberto Madail told reporters after signing the FIFA document with Spaniah counterpart Angel Maria Villar.
The winning bid will be announced in December 2010.
European champions Spain and Portugal declared their interest in hosting the tournament after FIFA said it would welcome bids for both the 2018 and 2022 World Cups.
FIFA president Sepp Blatter has said Spain, alone or with Portugal, were strong candidates to be hosts in 2018, as were England and the Netherlands with Belgium.
Australia, China, Mexico and Russia have also expressed interest.
Spain has not hosted the World Cup finals since 1982, but Portugal built 10 new stadiums to host the European Championship in 2004.
The 2010 World Cup will be held in South Africa while Brazil will host the 2014 tournament.