Schroders, which owns 14.6% of Provident's shares, is the first major investor to come out publicly in rejecting the bid.
Non-Standard's bid for Provident has the backing of Invesco, Woodford and Marathon Asset Management, who between them own more than half of the shares in the embattled doorstep lender.
All three are also investors in Non-Standard Finance, controlling two-thirds of the shares.
Schroders fund manager Kevin Murphy and global head of stewardship Jessica Ground took aim at the the three in a letter to Provident chairman Patrick Snowball.
'We do not believe the shareholders of Provident Financial who are also collectively majority shareholders in Non-Standard Finance (namely Woodford, Invesco and Marathon) should be seeking to impose the challenges of the latter company on the former,' they said.
'We are concerned that the rights of minority shareholders are not being protected, and this represents poor stewardship.'
Murphy and Ground said the proposed takeover of Provident Financial risked jeopardising a recovery from the catalogue of problems that have plagued the lender and sparked a 70% fall in its shares in 2017.
'Provident Financial has faced a number of issues in recent years, but the first quarter trading statement shows that it is on track with its recovery and rehabilitation,' they said.
'In our view Non-Standard Finance's bid risks destabilising this recovery, and brings additional risks and uncertainty.'