Duberry has seen only limited first-team action with Leeds recently (RossKinnaird/Allsport)

It is understood that Martin O'Neill attempted to land Duberry prior to the Champions League signing deadline last August and that the £3million-rated former England Under-21 defender has once again been the subject of a recent inquiry.

Duberry has been condemned to life on the Elland Road sidelines since the recent high-profile court case against his club-mate and former England international, Jonathan Woodgate.

Previously a target for Premiership clubs Southampton and Charlton, Duberry increased speculation over a possible move north by attending the recent Old Firm clash at Ibrox alongside Leeds teammates Lee Bowyer and Rio Ferdinand.

And, despite signing a new five-year contract before Christmas, the 26-year-old has not been a regular in the Leeds starting line-up.

A Duberry confidant revealed last night: 'Martin O'Neill is a long-term admirer of Michael. He tried to sign him for Leicester when Michael was at Chelsea and tried again to sign him for Celtic before the European-signing deadline. Another attempt wouldn't be a great surprise.'

Unless they enjoy a Mark Viduka summer sell-on bonanza, however, Celtic's ability to meet the Elland Road valuation of Duberry could hinge upon increasing pressure on the champions to sell Valgaeren.

Borussia Dortmund coach Matthias Sammer has already placed the highly-rated centre-half on his shopping list, whilst one prominent Premiership club has appointed a go-between to approach Celtic within the next fortnight - after learning of the defender's willingness to consider a move from the SPL.

Manchester United, Newcastle and West Ham are in the market for close-season defensive captures and a source close to the deal last night said: 'This is at a very delicate stage and no-one wants to say anything that could dissuade Celtic from selling the player.

'Dortmund are Valgaeren's first choice but there will be a bid from England going in within the next fortnight. There is a Premiership club very interested and, if the player starts to agitate, then it could be hard for Celtic to resist a £4m offer.'

O'Neill, meanwhile, will return from the Easter break tomorrow to take another look at colourfully named 18-year-old Djurgardens defender N'Diwa Lord-Kangana.

The youngster spent a week training at Barrowfield in October and will spend a further week being assessed after his agent, former Swedish international striker Martin Dahlin, said: 'It will be a good experience for him to play at a top foreign club. It'll do him the world of good.'