Red Bull must release Alexander Albon from his contract if the Thai is to return to Formula One with Mercedes-powered Williams next season, Mercedes team boss Toto Wolff said on Saturday.
Williams will need another driver if, as expected, young Briton George Russell moves to Mercedes next season alongside seven times world champion Lewis Hamilton.
"It's tricky to have a 100per cent Red Bull driver on a Mercedes power unit, so I'm happy to work with him as long as they let him free from his Red Bull contract," the Austrian told reporters at the Dutch Grand Prix when asked about Albon.
The 25-year-old Thai, dropped by Red Bull Racing at the end of last year but still under contract and now competing in the German Touring Cars (DTM), is a leading contender for the Williams seat.
Mercedes are also pushing their new Formula E champion Nyck de Vries, however, and Red Bull boss Christian Horner said on Friday he feared Mercedes would try and block Albon's move.
"Obviously there's a few hurdles. He (Albon) has had about four phone calls telling him not to go there from the team next door (Mercedes). I told him to ignore them," said Horner.
Wolff said Mercedes were backing de Vries but the Dutch driver was "his own free agent" in negotiations.
"Nyck is a Mercedes works driver in Formula E but I will never stand in Nyck's way to drive in Formula One," he said.
"We have no management contract with him, neither do we have an option of taking him back and therefore Nyck can drive in any team."
Ferrari-powered Alfa Romeo have a vacancy with Kimi Raikkonen's retirement, although Valtteri Bottas is expected to move from Mercedes to take it.
The other slot at Alfa is currently occupied by Italian Antonio Giovinazzi, who qualified an impressive seventh at Zandvoort on Saturday.
Alfa team boss Fred Vasseur has indicated he would rather have an experienced driver than a complete rookie, with major rule changes coming next year.
(Reporting by Alan Baldwin, editing by Toby Davis)