Mercedes are widely reported to have opted to retain Bottas for a fourth season ahead of Esteban Ocon, who is expected to make his racing return to F1 with Renault.
The Silver Arrows' team principal Toto Wolff confirmed to Gazzetta dello Sport that Hamilton would be consulted over the identity of his team-mate for next year.
The reigning world champion has regularly talked up his relationship with Bottas, which has brought relative calm to Mercedes after three years of fractious racing alongside Nico Rosberg.
Former Benetton and Sauber driver Lehto hopes that disappointing results in Germany and Hungary before the summer break do not count against Bottas, given his run of success alongside Hamilton previously.
Lehto told Iltalehti of reports suggesting Bottas would be announced imminently: "Before it is official you can't believe it, but I certainly hope it's true.
"He has done a good job and challenged Hamilton really well. He had a few bad races just before the summer break, but I think the decision was being made over a much longer time.
"The team will look at what combination of drivers is best for them. It certainly seems that Hamilton wants Valtteri to be the other driver, and that is a factor that will be considered."
Formula 1 return to racing this week at the Belgian Grand Prix, with Hamilton enjoying a mammoth 62-point lead over Bottas, his nearest challenger, in the drivers' standings as he chases a sixth world title.
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