The midfielder opened talks with the Clydesdale Bank Premier League champions less than 24 hours after his penalty miss helped send the club into the Co-operative Insurance Cup final at the expense of Dundee United.

Celtic agreed an undisclosed fee with Cardiff for the 23-year-old, who spent 18 months on loan at Tannadice.

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Flood hit out at Celtic over the "stupid" transfer speculation immediately after Wednesday's semi-final at Hampden but his "dream" move soon materialised.

The Irishman, who began his career at Manchester City, revealed his pride over signing for the Hoops.

"It really is a tremendous honour to sign for a club of Celtic's stature," Flood told www.celticfc.net.

"This is an exciting challenge and one which I am really looking forward to.

"I have been a fan of Celtic all my life and this truly is a dream come true to sign for the club.

"I will be joining a great group of players and I will be giving everything I can to make as big a contribution as possible to Celtic Football Club.

"I hope to play my part in achieving more and more success for the club and its supporters."

Flood, who made 68 appearances for the Tayside outfit, added: "I would like to thank Dundee United for all they have done for me.

"I have enjoyed my time there immensely and I wish the club all the best for the future. My focus is now Celtic and I can't wait to get started with my new club."

Manager Gordon Strachan said: "We are delighted to welcome Willo to Celtic.

"He has had a tremendous time at Dundee United, where he has shown great ability, drive and enthusiasm. We look forward to working with him at Celtic."

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New team-mate Gary Caldwell earlier hailed the "phenomenal" character Flood showed at Hampden.

Flood missed his second penalty of the sudden-death shootout before Scott McDonald wrapped up an 11-10 win for Celtic.

He admitted feeling unsettled by the speculation despite twice coming close to breaking the stalemate during the 120 minutes of normal action.

Caldwell played for Hibernian against Celtic after signing a pre-contract with the Glasgow club but he admitted Flood had faced a far more difficult test of his mettle.

"It was phenomenal," Caldwell said. "I did it myself in a meaningless league game a few years ago and to have to do it in a semi-final is different, and take penalties and everything else.

"That was excellent and shows what a good character is."

The Scotland defender added: "It's hard enough to take one penalty. It takes a lot of courage for the two players that had to take two.

"To have to go back up just puts more on your mind: do you put it the same way or do you change?

"It was hard on him but I thought he played excellently, as did a lot of players in their team."