Thompson takes over the entry occupied at Knockhill last month by Rob Huff and will see out the season this weekend at Silverstone and next month at Brands Hatch.
Reigning Jack Sears Trophy champion Thompson starred in the first half of this season, qualifying fourth at Oulton Park, but was forced onto the sidelines thereafter when 2020 JST title winner Michael Crees brought a budget to return to the BTCC.
Nic Hamilton then abruptly quit the team on the morning of qualifying for the following round at Croft, creating the vacancy that was filled by 2012 World Touring Car champion Huff at Knockhill.
After missing the interim round at Donington Park due to clashing TCR World Tour commitments, Huff was tipped to return for Silverstone and Brands.
The 43-year-old has decided instead to stand down but is weighing up a return to his domestic championship for next season, 20 years after his only full-time BTCC season as a tin-top rookie with the works SEAT squad in 2004.
“It was great for Tony [Gilham, Hard boss] to get me in the car at Knockhill, but unfortunately the stars haven’t aligned for the rest of the season,” Huff told Autosport.
“I will be attending Silverstone on Saturday to speak to teams about potential opportunities for next year for sure.
“We’ve moved back to the UK [from Huff’s former residence in Dubai] and the BTCC is something I’d love to do.
“It’s on my doorstep, it’s where I grew up, and it’s where I’d love to finish my career off.”
The second driver change of the season in the Hamilton/Huff/Thompson Cupra is against the letter of BTCC regulations regarding the TBL entrants’ licences.
These allow only one driver swap, but Autosport understands that approval from the other teams has allowed Team Hard an exemption.
Regarding the return of Thompson, Gilham said: “Bobby’s commitment to Team Hard Racing has been inspiring, and we have sorely missed his valuable contribution off-track since the summer break.
“We are delighted to have him back and believe that this reunion will propel us to even greater heights as we look to put the events of Donington behind us.”