CAIRO, Aug 29 (Reuters) - Egyptian authorities will detain former presidential candidate Ahmed Shafik for questioning over an alleged corruption case if he returns from abroad, a judge said on Wednesday.

Shafik, ousted President Hosni Mubarak's last prime minister, has been placed on a watch list at border points, Judge Osama el-Saeidy said in a statement.

Shafik, who lost a presidential run-off vote against the Muslim Brotherhood's Mohamed Mursi in June, left Egypt for Abu Dhabi two days after his opponent was declared president.

His aides have repeatedly denied Shafik is on the run, saying he would return soon to launch a new liberal political party, but the threat of arrest could deter him, analysts say.

"The investigative judge decided to put General Ahmed Shafik's name on the watch and travel ban lists based on investigations over his illegal allocation of 40,000 square metres of land... to Alaa and Gamal Mubarak," Saeidy said.

Mubarak's two sons Alaa and Gamal are now in prison facing corruption charges. Their 84-year-old father was sentenced in June to life in prison over the killing of protesters during last year's popular uprising that ended his 30-year rule.

Dozens of legal cases have been filed against Mubarak associates since the president fell on Feb. 11, 2011.

The one against Shafik was lodged in May by Essam Sultan, a leader of the Islamist Wasat Party, who accused the ex-airforce chief of exploiting his former position as head of the Young Air Force Officers Association to sell land owned by the association to Mubarak's sons at reduced prices.

Shafik denied the charge and accused Sultan of being a former state security informant who had been used to spy on Islamists during Mubarak's era.

Sultan told Reuters after Shafik was put on the watch list that he hoped his adversary would return "to face a fair trial and present evidence on the accusations he made against me".

One of Shafik's supporters, Ahmed Abdel Ghani, said the Muslim Brotherhood was behind the moves against Shafik.

"I warn them to not touch General Shafik when he comes back or else civil war will break out," he said. (Additional reporting by Tamim Elyan and Patrick Werr; Editing by Alistair Lyon)