LONDON A UK decision to leave the European Union would deprive the EU of one of its best-rated members, leaving the bloc and its investment bank vulnerable to credit rating downgrade.
While ratings agencies have warned a vote to leave could hurt the UK's rating, a Brexit could also have consequences for institutions which Britain supports via its EU membership.
Britain accounts for some 16 percent of the EU's nominal gross domestic product, makes a net annual budget contribution of about 6.5 billion euros ($7.2 billion) and is among the highest-rated of the EU's 28 members.
It is rated AAA by S&P, Aa1 by Moody's and AA+ by Fitch.
"European entities which would be most affected in the case of Brexit are the European Union itself, with a risk of being downgraded, and the European Investment Bank (EIB)," said Societe Generale strategist Cristina Costa in a note.
The EU, which has about 54 billion euros in outstanding bonds, is rated AAA by Fitch, Moody's and DBRS and AA+ with a negative outlook by Standard & Poor's.
Since the agencies have stressed the average weighted ratings in their approach to supranational borrowers, a British departure could weigh on the ratings of the EU and the EIB, analysts said.
No comment was immediately available from the EU.
There would be no immediate rating impact from Britain leaving the EU on any of the bloc's other members, Moody's said on Thursday.
Analysts say the EIB is especially at risk from a potential Brexit because while the EU and the EIB have similar memberships, they have different structures with regards to financing from its shareholders.
Unlike the EU, the EIB operates like a bank, so has its own capital base - about 243 billion euros, according to its latest financial report. The EIB is also able to tap additional funds known as callable capital, which is almost 222 billion euros.
Because ownership of the EIB is concentrated in the region's five largest economies, its profile would weaken if the UK left, analysts said.
The UK has a 16.1 percent share of the EIB's capital, similar to that of Germany, France and Italy, as this graphic shows tmsnrt.rs/1LT8zHX
While other member states would probably pick up a reduced UK share, the uncertainty could weigh on EIB bonds.
According to SocGen, the loss of Britain as a shareholder would lower the share of high-rated EIB backers to roughly 60 percent from 66 percent.
An EIB source told Reuters there was no automatic consequence of a Brexit on the EIB's AAA rating.
"If it were to materialise, the EIB would closely monitor any potential impact ... and would consider how best to address such a situation and reassure the markets in due course," the source said.
Commerzbank rates strategist Rainer Guntermann said any Brexit downgrade to the EIB's ratings could have implications for bond investors.
"Regulation-driven investors in the insurance sector, even if not forced to, prefer to hold triple A-rated paper such as from the German government, EIB or German agencies," he said. "There would be a significant impact if the EIB rating were to slide just one notch."
The gap between EIB bond yields and the European agency benchmark, KfW, has been tight with the five-year yield spread at about 6-8 basis points.
As the June 23 vote approaches, SocGen and Commerzbank anticipate wider spreads.
SocGen's Costa said spreads could widen by at least five to 10 bps even with support from the ECB's bond-buying programme.
Other institutions such as the European Stability Mechanism, the European Financial Stability Facility, and the European Bank for Reconstruction and Development, were not expected to take a direct hit.
(Additional reporting by Jan Strupczewski in Brussels; Editing by Ruth Pitchford)