A second investment manager is voicing its opposition to a plan by Ritchie Bros. Auctioneers Inc. to buy vehicle auction company IAA Inc.

In a letter to the Ritchie Bros. board, Janus Henderson Investors says it believes the deal would introduce a level of unnecessary risk for Ritchie Bros. shareholders.

The firm, which is an investment adviser to clients and funds that hold Ritchie Bros. shares, says IAA's business has been losing market share over the last several years and may be structurally disadvantaged compared to its main competitor, Copart.

The letter Monday comes after investment firm Luxor Capital Group LP said it is opposed to the deal.

Ritchie Bros. is offering US$12.80 per share in cash and 0.5252 of a Ritchie Bros. share for each IAA share. It also plans to pay a special one-time dividend of US$1.08 per share to its own shareholders, contingent on the deal closing. 

Ritchie Bros. says many of its shareholders support the deal and it will continue to engage with Janus.

The deal, which has been approved by the boards of both companies, still requires approval by the shareholders of Ritchie Bros. and IAA.