CELTIC are investigating claims that a film showing two of its players taking part in a drunken sing-song was overdubbed with sectarian chants.

The Sunday Herald understands that the club have had footage of striker John Hartson and winger Stephen Pearson analysed and believe that the clip was doctored to include shouts of "IRA!" and "Sinn Fein!"

Fury erupted last week after it emerged that the two international footballers were filmed at a late-night party with fans who appeared to shout support for the Irish terrorist group during pauses in a rendition of the club anthem, The Fields Of Athenry.

Politicians attacked the players over the footage and anti-sectarian charity Nil By Mouth said it had written to Celtic demanding that the club investigate the matter. But last night sources revealed that the club had received word that the footage, which first came to light after it was posted on football supporters' websites last week, had the sectarian shouts added later. The sources also said that the club believed that the copy was "not of a high quality".

The incident happened at a testimonial party in Donegal in Ireland for former Celtic captain Jackie McNamara. Hundreds of fans turned up alongside players and backroom staff at the fourstar Clanree Hotel in April to pay tribute to the Scotland star.

In the grainy film clip, taken on a mobile video phone, Pearson is seen joining in with the Irish folk tune as revellers in the background shout sectarian slogans during pauses in the song. Hartson, wearing a black striped shirt, is also shown on the film hugging fans and bouncing around. Both players have admitted singing along to the terraces' favourite, but have strongly denied any IRA shouts.

The claims that two high-profile players were caught up in a sectarian storm are especially embarrassing for Celtic, who, along with Rangers, have been at the forefront of anti-sectarian drives. The club has also stated its opposition to the promotion of paramilitary organisations.

Last night a source suggested that it was easy to doctor film clips. He said: "You can buy software for around pounds-30 that can allow you to manipulate sound and images on video phones. It does not take much technical expertise to overdub sound on these things."

A Celtic spokesman would only say that the club had launched an inquiry into the possibility that the players had been involved in the anti-sectarian shouts. A spokesman said: "We are investigating various aspects of this. I am not in a position to comment further."

Donald Gorrie, the Liberal Democrat MSP who campaigned for antisectarian laws, said: "The clubs have to lead on putting out the message that enthusiastic supporters are a good thing, but there has to be a limit."

Patrick Sweeney, who organised the function, said: "The players were a credit to Celtic football club that night. I don't recall IRA chants."

Both the players' agents have rejected any claims of wrongdoing. Yesterday Celtic also said that any suggestion their players had taken part in sectarian behaviour was "utterly laughable".

The spokesman added: "The club and players are both considering their legal options. Both players freely admit they joined in the singing of The Fields of Athenry, a well-known and popular Irish folk song, at a supporters' event in Ireland. But they absolutely did not join in any sectarian chanting."

A spokesman for Nil By Mouth urged Celtic to fully investigate the matter and publicise its findings.