Paul Hartley admits the signing of Georgios Samaras has boosted the Celtic players following his impact on the club's successful title push.The Greece striker, who scored six goals despite a limited number of starts after his January loan signing, completed a permanent transfer from Manchester City on Tuesday.

Samaras will now be keen to dislodge Scott McDonald and Jan Vennegoor of Hesselink from the starting line-up after pledging his future to Celtic for three years.

"He made a big impact when he came into the team," Hartley said.

"He scored a lot of important goals for us, so he is going to be a big player for us again.

"He enjoys his football here. He probably had other offers or he could have stayed at Man City but he wanted to come here and show what he can do."

Samaras is understood to have taken a £10,000 cut in his weekly wage to sign for the Scottish champions after taking to the "family" atmosphere at Parkhead.

And former Hibernian and Hearts midfielder Hartley, who signed for Celtic 18 months ago, agrees that the atmosphere is unique.

"This club is special, there is no doubt about that," the Scotland international said.

"To play for a club the size of Celtic is a fantastic feeling, and as Samaras said, we have got a fantastic team spirit here.

"We all work hard together, try and win as many football games as we can. There is no better place to play your football.

"Every second week you are playing in front of a full house of 60,000, so why would you want to play anywhere else?"

The team spirit Hartley rates so highly helped Celtic to an unlikely Clydesdale Bank Scottish Premier League title win last season.

Gordon Strachan's men won their last seven games to lift the trophy on the final day after consecutive defeats by Rangers and Motherwell appeared to have ended their hopes.

Hartley said: "Everybody had written us off probably four or five months before the end of the season but we knew as a group of players that we were very strong mentally, and knew if we could keep a run of games going that we could win the championship. That's the way it ended up.

"This season is another new challenge again because everyone will have a stronger squad.

"It's not just Rangers who are adding players. Dundee United will be very strong again, Aberdeen, maybe Hearts will be a bit stronger too.

"So we know we have got another fight on our hands as well."

It is a battle that Hartley goes into with confidence despite few additions to the Celtic squad.

With Samaras having been at Celtic since January, only Derry City winger Pat McCourt has come into the set-up for the first time.

Parkhead officials are bidding to secure a deal with Auxerre and Romania defender Gabriel Tamas over a £3.5million transfer.

But Hartley is sure they have the strength in depth and attitude to mount a challenge for a fourth successive title.

"We have got a winning mentality at this club and a lot of good players," the 31-year-old said.

"It's always important to add one or two players to your squad to freshen things up and keep people on their toes but we have a very strong squad here, as we saw at the end of the season.

"It will be another difficult season because other teams have added to their squad so it's going to be hard again.

"All we can do is focus on what we do and hopefully do it again."

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