Swiss Re has agreed to acquire UK-based Guardian Financial Services for £1.6bn.
Guardian is closed to new business but continues to manage its portfolio of annuities, life insurance and pension policies. It manages around £18bn of assets on behalf of 900,000 customers.
Both Guardian and Admin Re are in the business of consolidating existing books of life insurance.
The deal will see Guardian's 900,000 policyholders in the UK and Ireland added to Admin Re's 3.4 million policyholders to create a portfolio of more than four million customers. It will also increase Swiss Re's assets under management by £12.5bn, or 15%.
Zurich-listed Swiss Re's shares were today trading at 83.70 Swiss francs (£56.44). It has a market value of around 31bn Swiss francs (£20.9bn).
Swiss Re said in a statement the acquisition would help Admin Re become a leading closed life book consolidator in the UK.
Michel M Liès, Swiss Re's group CEO, said: "This acquisition is an excellent opportunity for Admin Re to further enlarge its successful business and diversify its portfolio. It is proof that we can deliver on our ambitions to seek profitable growth opportunities for Admin Re in the UK.
"The expected returns exceed our profitability targets for new business and represent an excellent fit with our group strategy as well as with Admin Re's capabilities and existing infrastructure."
Ian Owen, executive chairman at Guardian, said: "I'm delighted that the next chapter in Guardian's long history will be as part of one of the leading closed book consolidators in the UK. Admin Re has a proven track record in integrating life portfolios onto its platform while maintaining a high level of customer service for policyholders."
Guardian has two subsidiaries: Guardian Assurance and Ark Life Assurance. The former was closed to new business in 2001. Dublin-based Ark Life Assurance was closed to new business in 2012.
Last week another closed fund consolidator Phoenix announced it was in talks to take over Guardian, but the deal did not go ahead.
Credit rating agency Standard & Poor’s said the acquisition would not affect Swiss Re's ratings.
It said: "We consider this acquisition to be in line with Swiss Re's strategy to place Admin Re as a leader in the closed life book market, with particular focus on the UK. We expect the group's capital adequacy to be sufficient to absorb this acquisition and remain extremely strong over the next three years."
The deal is expected to be completed in early 2016, subject to regulatory and shareholder approval.