TORONTO, March 28 (Reuters) - Independent advisory firm ISS has urged shareholders of Lundin Mining <LUN.TO> shareholders to vote against the company's proposed acquisition by Canadian rival Inmet Mining <IMN.TO>.

ISS had initially recommended that Lundin shareholders back the proposed deal, which would lead to the creation of a new base metal miner called Symterra.

However, the proxy advisory firm has now altered its stand, arguing that Lundin's shares have traded at a premium to the Inmet offer since rival Equinox Minerals <EQN.TO> came in with a competing C$4.7 billion ($4.8 billion) bid for Lundin late last month.

Since Feb. 28, Lundin's shares have traded at a slight discount to the Equinox offer, but at an average premium of 16.6 percent to the implied Inmet offer price, suggesting that the market believes Equinox is more likely to be successful, ISS said in a note issued on Friday.

The firm also noted that there was some uncertainty around Inmet's $4.3 billion Cobre Panama copper-gold project in Panama. The government in Panama, which recently announced plans to repeal certain mining laws, has now indicated it may not approve a coal-fired power plan to for the project, ISS said.

Earlier on Monday, ISS advised Equinox shareholders to back its board's proposed share issue plan in connection with the company's bid for Lundin. (Reporting by Euan Rocha; editing by Rob Wilson)