LONDON (AFP) - A Manchester City youth player will return to prison after mistakenly being released on Monday four months into a 16 month sentence for killing a brother and sister while speeding in his Mercedes in central Manchester.

Britain's Prison Service said it would hold an investigation after 18-year-old Courtney Meppen-Walter was released in error.

"Police have been informed after a prisoner was released in error on Monday 1 July to ensure he is promptly returned to custody," said a spokesman.

"Releases in error are very rare and we take any incident extremely seriously. "An investigation will take place into the circumstances that led to this release." Meppen-Walter was driving at nearly double the speed limit of 48kmh when he hit a car carrying Kulwant Singh, 32, and his 37-year-old sister, Ravel Kaur.

Both siblings, who hailed from the Manchester area, were killed, while Kaur's two sons, aged 16 and 17, were injured.

Their relatives said they felt "let down" by Monday's mistake.

Meppen-Walter, who was also the captain of England's under-18 side, is due to serve half his sentence in custody but may be released earlier and obliged to wear an electronic tag.

The judge also banned him from driving for three years and ordered the sale of the car, with the proceeds to go to the injured passengers.