Michael Shields and Anthony Wilson, both 18 and from Liverpool, had gone on to a Bulgarian resort after the game.

They were due to fly back to the UK on Sunday but were arrested along with two others by local police on suspicion of attacking a man a few hours earlier.

The alleged victim is believed to have been hit over the head with a stone.

One of the two other men was later released without charge while 19-year-old Bradley Thompson, of Liverpool, was given bail but told to stay in the country.

This is supposed to have happened at 5am on the Sunday morning, but Michael and his friends went to bed at 2.30am Michael Shields senior

Mr Shields and Mr Wilson appeared at Varna District Court on Friday where a judge granted permission for them to be detained for a further 72 hours.

They face a possible charge of attempted murder.

Mr Shields' father, also called Michael, said his son was innocent, adding: "This is supposed to have happened at 5am on the Sunday morning, but Michael and his friends went to bed at 2.30am.

"They were all in the same room and his friends have made sworn statements that Michael was with them.

"He's a good lad. He's never been in trouble with the law and he was training to work on the railways.

Fly to Bulgaria

"This is worse than a nightmare for our family, but we'll do everything in our power to prove Michael's innocence."

He added that he would be flying to Bulgaria to support his son.

A spokesman for the Foreign Office said: "The two British men have not been charged with anything. They appeared in court and a judge granted an extra 72 hours so that the investigation may continue.

"A third man has been given bail but is not allowed to leave Bulgaria."