MIDDLESBROUGH chairman Steve Gibson last night told Newcastle counterpart Freddy Shepherd he would have to dig deep to sign striker Mark Viduka.

The pair spoke yesterday to discuss a potential swap deal that would see Viduka leaving the Riverside for St James' Park, with midfielder Nicky Butt travelling in the opposite direction.

But while Boro have grudgingly accepted that Viduka may be surplus to requirements next season, they remain adamant they will not allow the Australia international to leave on the cheap.

Gibson will only sanction Viduka's sale if he receives at least the £4.5m he paid for the 29-year-old last summer.

That could yet put a halt to the deal as Shepherd, who remains confident of completing the transfer of Fenerbahce striker Nicolas Anelka by the end of the week, is reluctant to match that sum.

The Magpies already have a £4m bid on the table for Anelka and a £3.5m bid lodged with Fulham for midfielder Luis Boa Morte. Assuming both of those deals can be completed satisfactorily, that would take Newcastle's summer spending to more than £18m and leave little surplus cash for a move for Viduka.

Newcastle officials first inquired about the former Leeds striker two weeks ago, but were initially told that Boro boss Steve McClaren did not want to lose him.

That situation has subsequently changed as McClaren has been unable to sell any of the eight strikers currently on his books. Massimo Maccarone's proposed move to Siena collapsed at the last minute, while Wigan have failed to follow up their initial interest in Szilard Nemeth and a number of French clubs have turned down the chance to sign Joseph-Desire Job.

Viduka was on the bench for the first game of last weekend's four-team tournament on the Algarve but, significantly, was withdrawn from the Boro squad ahead of Saturday's third-place play off with Real Betis.

McClaren is now willing to let Viduka leave but, like his chairman, believes the striker has lost none of his value in the last 12 months, despite playing just 12 minutes of club football since the turn of the year.

Newcastle's reluctance to shell out £4.5m led them to raise the prospect of a swap deal involving Butt yesterday.

The former Manchester United midfielder failed to sparkle at St James' Park last term and his fate was effectively sealed when he came in for sustained criticism following an abject display in April's FA Cup semi-final defeat at Old Trafford.

McClaren has spent most of the summer trying to sign a central midfielder to supplement George Boateng and Doriva. The Boro boss thought he had signed Stephen Appiah, only for the Ghana international to move to Turkey instead, and has also made an unsuccessful attempt to entice Owen Hargreaves from Bayern Munich.

He formed a close relationship with Butt during his time as assistant to Sir Alex Ferguson at Manchester United and remains an avowed admirer of the 30-year-old.

But it is understood he does not value him at any more than £2m, leaving a £2.5m shortfall that Boro will demand is met in cash. Newcastle paid £2.5m for Butt's services last summer and, given the midfielder's fraught first season on Tyneside, it is difficult to see how Magpies officials can dispute McClaren's valuation.

Indeed, it is difficult to see how Newcastle's board can force McClaren's hand at all given the Magpies' critical striker shortage ahead of the new campaign. For the third game in a row, Souness' side will go into tomorrow's Intertoto Cup semi-final second leg against Deportivo La Coruna with just Alan Shearer and Michael Chopra to lead the line.

Further talks are planned this week, with Viduka expected to play no more than a peripheral role when Boro take on Spanish side Atletico Madrid in a pre-season friendly at the Riverside tomorrow.

Shepherd is also expected to make further contact with both Fenerbahce and Fulham, but insists he will not up his offer for either Anelka or Boa Morte.

The United chief has also spoken to Celta Vigo in order to give the Spanish side permission to speak to want-away Newcastle midfielder Hugo Viana.

"We gave Hugo permission to speak to Celta Vigo and that is what he's doing," said Shepherd.

"Nothing has been agreed. He's in Spain talking to them and we are waiting to see what happens."

Viana was due to appear for Newcastle reserves at Hartlepool on Friday night, but was pulled out of the squad which travelled to Victoria Park.

Read more about Middlesbrough here.