Thatcher won seven caps for Wales

The 31-year-old is set to replace Djimi Traore at left-back after the Mali international moved to Portsmouth earlier on Thursday.

"We are going to need a tougher, harder edge if we are going to get out of this," said Charlton boss Alan Pardew.

"I looked at Ben, saw he was available and that he would improve us."

Thatcher said that manager Alan Pardew was a big factor in his decision to join the club.

"I am looking forward to the challenge of helping to keep Charlton in the Premiership this season," said Thatcher.

"It is a fresh start for me and I am looking forward to working with Alan Pardew, who I have always respected as a manager.

"We have got a massive game against Middlesbrough on Saturday and I hope that I can play my part and help the team to a big three points."

Thatcher hit the headlines earlier this season after he received an eight-game ban for his elbow on Portsmouth midfielder Pedro Mendes.

"Ben is a committed player, and fans know he will put a tackle in fairly and play good football for Charlton," said Pardew.

"I am going to handle him the same way I have done others who have come to me with similarly bad disciplinary records. I think Lee Bowyer has had one booking so far this season when I was at West Ham.

"You just have to be very stern and strict about it and make sure there are guidelines to what they can and can not do.

Ben is going to be the sort of character we are going to need Charlton boss Alan Pardew

"We have conceded more goals than anybody else, and I feel we need a bit more dialogue in the back four.

"Ben brings great experience and spirit to the team, and strength. He is going to be the sort of character we are going to need."

Meanwhile, City boss Stuart Pearce said he wanted to strengthen other areas of his squad with the proceeds of Thatcher's sale.

"The finance we gain can be invested in other positions," he said.

"I believe I have enough cover to cope in his absence."

Thatcher has slipped down the pecking order at Eastlands with Stephen Jordan establishing himself as first choice at left-back.

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Centre-back Sylvain Distin is also able to provide cover on the left side of defence.

The former Wales international started his career at Millwall and has since had spells at Wimbledon, Tottenham, Leicester and Manchester City.