Red Bull team boss Christian Horner lost a number of personnel to Aston Martin in recent months
Aston Martin have produced an extensive upgrade of their car for this weekend's Spanish Grand Prix which bears a striking resemblance to Red Bull's design.
Red Bull F1 boss Christian Horner said the idea their intellectual property might have been used was of "grave concern".
Aston Martin said the FIA had confirmed it was "legitimate independent work".
Horner's comments came before the FIA released a statement clearing Aston Martin, saying an investigation had confirmed "no wrongdoing had been committed, and therefore the FIA considers that the Aston Martin aerodynamic upgrades are compliant".
But a Red Bull statement issued after the FIA's response made clear they felt the matter was not yet over.
It said they "noted the FIA's statement with interest", adding: "Should any transfer of IP have taken place that would clearly be a breach of regulations."
Aston Martin have been struggling close to the back so far this season, while Red Bull are contesting the world championship with Ferrari.
The upgraded Aston Martin features very similar bodywork design to Red Bull's, with heavily undercut sidepods and a stepped, ramp design on the top bodywork.
Horner, whose car has sported that design since the second pre-season test in early March, said he was concerned because Aston Martin had poached a number of Red Bull staff members in recent months.
Among them is Dan Fallows, Red Bull's former head of aerodynamics who was signed as Aston Martin's new technical director last year but took up his post only in April after a settlement between the teams.
"Copying is the biggest form of flattery," Horner said. "It is quite a thing to instruct your team to come up with a very close-looking clone of our car and of course a few people have moved over the winter period, and what you can't control is what they take in their heads.
"But what would be of grave concern to us would be if any IP had in any way changed hands.
"That is where we rely on the FIA to do their job, they research, they have all the access and we will be relying on them heavily to ensure that no Red Bull IP has found its way into that car."
An Aston Martin spokesperson said: "We have shared details of our update with the FIA technical people.
"Having analysed the data and the processes used to create the update, the FIA has now confirmed in writing that our update was generated as a result of legitimate independent work in accordance with the technical regulations."
An FIA statement said it had "carried out a routine pre-event legality check" into the Aston Martin before the Spanish race weekend, focusing on "in particular the topic of 'reverse engineering' and potential illicit IP transfer".
It said both teams had "collaborated fully" but that the investigation "confirmed that no wrongdoing had been committed, and therefore the FIA considers that the Aston Martin aerodynamic upgrades are compliant" with the regulations.
The FIA said the upgrade was "consistent with" the rules that forbid IP transfer between teams.
It is not the first time Aston Martin have caused controversy by apparently copying a rival's car.
In 2020, when they were competing as Racing Point, they were accused of producing a facsimile of the 2019 Mercedes.
At first, the FIA cleared the design, but after protests from rival teams Racing Point were docked 15 points and fined 400,000 euros for breaking the rules and illegally copying the brake ducts of the Mercedes.
-- Courtesy of BBC Sport