(Reuters) - British insurer Rothesay Life has agreed to buy 6 billion pounds of annuities from the UK arm of Dutch insurer Aegon (>> AEGON), its third such deal in the past 12 months.

The deal, announced on Monday alongside a 42 jump in Rothesay's full-year pretax profit, gives the company control of two thirds of Aegon's annuity portfolio in Britain while the remainder, worth about 3 billion pounds, is expected to be sold in a separate process, a source close to the matter said.

Aegon, advised on the deal by U.S. investment bank Citi, said its intention was to fully divest its UK annuity portfolio, while Rothesay Life, whose backers include Goldman Sachs (>> Goldman Sachs Group Inc), Blackstone (>> Blackstone Group LP) and Singapore's sovereign wealth fund GIC, said it expected to do more deals in the future.

Some general insurers such as Aegon are looking to reduce their exposure to UK annuities - financial products which provide a guaranteed income for retirement - after recent reforms have given Britons more freedom on how they can use their pensions savings, hitting demand for annuities.

Specialist insurers such as Rothesay, meanwhile, are seeking to fill the breach and pick up assets at attractive prices.

The acquisition of Aegon's annuities, the value of which was not disclosed, could pave the way for an initial public offering (IPO) of Rothesay in about 12 months, the source said.

Rothesay said in a statement that "while the shareholders do see listing the business as a potential option for Rothesay Life, our current focus is on growth opportunities and our existing shareholders fully support this plan as evidenced by this transaction."

The specialist pensions insurer also said it had boosted its full-year pretax profit thanks to an increase in new reinsurance and corporate pensions deals.

It posted 347 million pounds in pretax profit in 2015, after new business volumes grew to 3.5 billion and assets under management rose 20 percent to 15.4 billion.

"We believe that the trend of multi-line insurers looking to reduce their exposure to annuities will continue and provide further opportunities for us to accumulate assets and grow the business," Chief Executive Addy Loudiadis said.

The London-based insurer bought a 1.2 billion pounds annuity book from Zurich last year, while in January it agreed to take on another tranche of the Swiss group's annuities. It previously secured control of Britain's MetLife Assurance in 2014.

It competes with specialist providers and larger insurers such as Legal & General (>> Legal & General Group Plc) and Prudential (>> Prudential plc).

(Additional reporting by Simon Jessop in London; Editing by Sunil Nair and Mark Potter)

By Noor Zainab Hussain and Pamela Barbaglia