BlackRock, the US asset manager, will pay UBS $15bn for a portfolio of subprime mortgage debt in a deal that the Swiss bank plans to announce today with its first-quarter results, people familiar with the transaction said.

The debt - which is being bought by BlackRock at a 25 per cent discount to its face value of $20bn - will be placed in a new fund and marketed to investors, the people said. UBS will hold a minority interest in the fund, which will enable it to participate in any potential upside.