The 31-year-old singer is said to be delighted with the news and has accepted the offer, reported Daily Mirror.
"Sarah is delighted. She's proved herself as an actress and comes across as sparky, bright and opinionated. Of course she's also very easy on the eye and will inject some much-needed glamour to the BGMT cast.
"She's been offered the gig on the night of the final which, obviously, will be the biggest TV audience of the series so far," a source said.
"Sarah has made no secret of the fact she'd love to be a judge on the 'X Factor'. She has a proven track record within the industry and knows the business. This BGT opportunity is seen as a fantastic way in and who knows what will happen in the future," the source added.