Vanguard Group has bid $5 billion (3.1 billion UK pounds) for rival Barclays' iShares division, reports the UK's Telegraph newspaper here.

Would be a hugely transforming move for Vanguard, which became the No. 1 fund group mostly by organic growth and hasn't had to manage mergers or take on huge debt. Vanguard spokeswoman Rebecca Cohen told me the company's not commenting, beyond noting the financial press has speculated for months about a possible deal.

"Vanguard, which is headquartered in Pennsylvania and had about $1trillion under management at the end of last year, is understood to have lodged the offer with Barclays in recent weeks. It is being advised by William Blair & Company, a Chicago-based investment banking group," the Telegraph reported.

"The offer from Vanguard underlines bidders' continued interest in iShares, despite Barclays' decision to begin talks with potential suitors for the whole of Barclays Global Investors (BGI), the operation within which iShares sits." Barclays has until June 18 to solicit rival offers, the Telegraph added.