CORRECTED-UPDATE 1-UK judge blocks $15 bln transfer of Prudential annuities to Rothesay Life

(Corrects paragraph 8 to say that Rothesay is already Britain's largest specialist annuity insurer)

* Prudential says demerger plans not impacted

* Both companies can appeal the verdict

* Prudential shares down 0.7%, among top FTSE 100 fallers

By Simon Jessop

LONDON, Aug 16 (Reuters) - A UK High Court judge has blocked the transfer of 12 billion pounds ($15 billion) in annuities from insurer Prudential to Rothesay Life, the two companies said on Friday.

The deal, which would have been the largest ever such transfer covering 400,000 policy holders, was announced in March last year at the same time Prudential said it was to demerge its UK business.

In response to tougher European Union rules introduced after the financial crisis, insurers have increasingly looked to free up capital by offloading closed books of annuity business to specialist firms such as Rothesay and Phoenix Group.

The details of the ruling were not immediately available, but are likely to lead to questions about whether other similar big deals in the sector can be struck. At 1329 GMT, Prudential shares were down 0.7%.

Prudential, which can appeal the decision, said it would have no impact on the company's planned demerger of its UK insurance and asset management business, set to complete in the fourth quarter.

The insurer said it was "disappointed" by the verdict.

"The Independent Expert, who was appointed to report to the High Court, concluded the transfer would have no material adverse effect on the security of benefits or the reasonable benefit expectations of our policyholders."

Rothesay, whose investors include U.S. private equity giant Blackstone and Singapore sovereign fund GIC, is Britain's largest specialist annuity insurer.

It said the deal had already been approved by regulators the Financial Conduct Authority and the Prudential Regulatory Authority.

"Whilst it is not the preferred or optimal outcome for either party, it will not have a material impact on Rothesay Life as a whole," it said.

($1 = 0.8222 pounds) (Reporting by Simon Jessop; Editing by Rachel Armstrong and Mark Potter)