Adds details from the FT report and background throughout and Castlelake response in last paragraph

June 22 (Reuters) - EasyJet EZJ.L and its investors are holding out for at least £600 million ($794.46 million) more from U.S. suitor Castlelake, the Financial Times reported on Monday, citing leading shareholders and people with knowledge of the talks.

The airline rejected the U.S. investment firm's $6.3 billion bid, saying it undervalued the business. Castlelake made the offer public earlier in the day.

"I think they'll engage if the price is at seven plus," the FT reported, citing a large unnamed investor, referring to an offer of at least £7 a share that would value easyJet at about £5.3 billion.

Castlelake said in a statement earlier on Monday that easyJet's "unwillingness to engage meaningfully" was a reason for going public with the bid, which included a partial equity alternative for investors.

Castlelake declined to comment on the report, while EasyJet did not immediately respond to a Reuters requests for comment.

($1 = 0.7552 pounds)