Manchester City look set to wrap up the signing of Aymeric Laporte but Arsenal's move for Pierre-Emerick Aubameyang hinges on other transfers being agreed as Premier League clubs scramble to beat Wednesday's deadline.
Spanish club Athletic Bilbao posted a leaving message from Laporte on their Twitter feed on Monday, saying his 65 million euros ($80 million) release clause had been paid and reports said he was due to travel to Manchester.
It means Pep Guardiola's Premier League leaders are set to surpass the Â£55 million they paid for Belgium attacking midfielder Kevin de Bruyne -- Laporte's transfer fee translates as Â£57 million.
The 23-year-old will also become the second most expensive defender in history behind Virgil Van Dijk, who cost Liverpool Â£75 million from Southampton earlier this month.
Laporte thanked Athletic's fans in his Twitter message but said he was taking on "a new challenge in my professional career".
"I don't want this to be a goodbye but rather a see you soon," he said. "Here, I'm leaving a unique and different club and I'll never forget it."
His signing will mean Guardiola has spent more than Â£200 million simply on reinforcing his defence alongside the acquisitions of Benjamin Mendy (Â£52m), John Stones (Â£47.5m) and Kyle Walker (Â£45m).
A France international at all youth levels, Laporte has been frustrated at continually being overlooked by senior team coach Didier Deschamps.
Borussia Dortmund striker Aubameyang is poised to make a Â£60 million switch to Arsenal, who are looking to bolster their forward options after the departure to Manchester United of Alexis Sanchez.
The BBC said Arsenal had agreed a fee with the Bundesliga club and personal terms with Gabon international Aubameyang, 28, but the German club will only sanction the move if they get a replacement.
Arsenal striker Olivier Giroud is reportedly a key cog in the transfer -- either as a direct replacement for Aubameyang or as part of a three-way deal, with Giroud moving to Chelsea and Michy Batshuayi swapping Stamford Bridge for Dortmund.
Reports in Italy suggest a Chelsea bid for Roma's former Manchester City striker Edin Dzeko has been unsuccessful.
Elsewhere, Liverpool striker Daniel Sturridge, 28, left Anfield on a loan deal until the end of the season after slipping down the pecking order under Jurgen Klopp.
Barcelona winger Gerard Deulofeu has become Javi Gracia's first signing as Watford manager after arriving at Vicarage Road on loan until the end of the season.
Millwall re-signed veteran Australia international Tim Cahill, 14 years after he left the Den to join Everton.
"It's hard to put into words (what it feels like to wear a Millwall shirt again) -- I'm still taking it in now," the 38-year-old told Millwall's website.
Celtic have signed Chelsea midfielder Charly Musonda on an 18-month contract, with left-back Baba Rahman departing Stamford Bridge for Schalke in a similar arrangement.
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