* Portugal's Expresso publishes "presidential order"
* No immediate comment from Angola's presidency, government
* Guarantee covers 70 pct of loans for 18 months, BESA says
LISBON, July 19 (Reuters) - Angola's president personally ordered that the state guarantee up to $5.7 billion in troubled loans belonging to the Angolan unit of Portugal's financially compromised Banco Espirito Santo, a Portuguese newspaper said on Saturday.
BES, one of Portugal's best-known banking institutions, is struggling to contain a crisis which erupted after an audit into its major shareholder Espirito Santo Financial Group revealed irregularities there, leaving a network of other companies controlled by the Espirito Santo family under scrutiny from investors and regulators.
The scale of those problems - and potential losses linked to unpaid debt - is not yet clear, but the fallout has spread to Portugal's former colony Angola where BES's 12-year Angolan unit BESA is a major financial player with links to the ruling elite and the dos Santos family.
On Saturday Portugal's weekly Expresso published what it said was a copy of the "Internal Presidential Order Number 7" signed by Jose Eduardo dos Santos on Dec. 31, 2013.
The document, whose authenticity could not be independently verified by Reuters, instructed Angola's finance minister to issue the guarantee for BES Angola (BESA) for up to $5.7 billion with the aim "to protect fundamental interests for the equilibrium of the Angolan financial system."
While BESA has already said that an Angolan state guarantee covered $5.7 billion, or 70 percent, of its loans, the fact that the guarantee was ordered by the president revealed his direct involvement in the move to protect the Angolan bank.
The presidential order noted BESA had a "relevant" loan portfolio financing Angolan companies, from small-sized to large, which were helping to advance the government's 2013-2017 National Development Plan seeking to boost the non-oil sector of the Angolan economy, which relies heavily on oil revenues.
Many banks in Africa's No.2 oil producer have been hit by a surge in bad loans in recent years due to an economic slowdown. The IMF sees Angola's real GDP growth slowing this year to 3.9 percent from 4.1 percent in 2013 and 5.2 percent in 2012.
The two-page document published by Expresso did not say how long the Angolan sovereign guarantee for BESA would be valid. But it said "all doubts and omissions resulting from the interpretation and application of this document will be resolved by the president", without elaborating further.
Angola broke its silence on the BES affair this week, when Angolan central bank governor Jose de Lima Massano admitted to parliament on Thursday there were problems with BESA's credit portfolio that included bad loans. He provided no figures and made no mention of the guarantee.
Asked about the guarantee on Saturday, Angolan presidency spokesman Aldemiro Vaz da Conceicao, who on Friday redirected questions to the central bank and finance minister, told Reuters "this has nothing to do with the presidency".
But when told Expresso had published what it said was a copy of the Angolan presidential order giving instructions for the guarantee, he said he would double-check. He did not then respond to follow-up emails and calls made over several hours.
Banco Nacional de Angola spokeswoman Amelia Borja Neto told Reuters in an email on Friday that questions about the sovereign guarantee should addressed to the "emitting entity", indicating it was not the central bank in this case.
Bank of Portugal Governor Carlos Costa said the Angolan guarantee helped to eliminate uncertainty around BES that holds a 55.71 percent stake in the Angolan affiliate.
In December, Ricardo Espirito Santo Salgado, then CEO of BES and patriarch of the Espirito Santo banking family whose business empire is now crumbling, met the Angolan president and Salgado has said the guarantee was agreed at that time.
Dos Santos, in power since 1979, is one of Africa's longest serving leaders, and led his ruling MPLA party through a long civil war that ended in 2002. The MPLA has won successive elections but rights groups and transparency campaigners say dos Santos' rule is personalized and autocratic and does not do enough to share out the country's oil wealth.
(Additional reporting by Pascal Fletcher in Johannesburg; Editing by Sophie Walker)