Mirror-image Ruud van Nistelrooy penalties proved too much for Portsmouth as Manchester United scrambled into the FA Cup fourth round at Old Trafford this afternoon.

The £18.5million Dutchman twice stepped up to hit the same corner, sending Shaka Hislop the wrong way on both occasions as Pompey's brave fightback was snuffed out with a late flurry.

Van Nistelrooy's first effort, quickly followed by a David Beckham free-kick, had threatened to sweep the Nationwide League promotion chasers away inside the first 17 minutes.

But Steve Stone's close-range effort set up a dominant second-half display from the visitors.

However, after chances to equalise had gone begging, van Nistelrooy stroked home his 20th goal of the season nine minutes from time before substitute Paul Scholes wrapped up a flattering win with an impish stoppage time chip.

Over the Christmas period, Gerard Houllier and Claudio Ranieri have both fallen foul of their desire to give all their first-team squad some match time, finding wholesale changes sometimes rob teams of the continuity which has brought them positive results in the past.

Sir Alex Ferguson is an old hand at the tactic though and, despite making seven alterations to the side who came back from the dead against Sunderland on New Year's Day, still turned out a team packed full of experience and capable of producing a devastating opening burst which threatened to dump Portsmouth out of the FA Cup before the competition had even begun.

It took United just four minutes to open their account against Harry Redknapp's Nationwide League high-flyers, by which time the visitors should already have been behind.

Shaka Hislop needed to make a smart save to deny van Nistelrooy as he took Kieran Richardson's lay-off in his stride but the former West Ham keeper was left stranded the next time he faced United's Dutch master.

Redknapp must have thought his side had snuffed out the danger when four defenders crowded round van Nistelrooy on the edge of the visitors' box.

Somehow, the former PSV man squirmed clear and laid a pass off to Ryan Giggs, whose first touch drew a rash challenge from Linvoy Primus, leaving referee Mike Riley with a straightforward penalty decision.

Van Nistelrooy exuded confidence as he strolled forward to take the kick, calmly sending Hislop the wrong way in most predictable fashion.

Even the backing of 9,000 travelling fans could not settle Portsmouth down and in the following half hour it was only United's poor finishing which prevented the visitors from being completely blown away.

Dealing with Beckham's dead ball delivery was the most glaring problem Pompey were encountering.

From corners and free-kicks, the England skipper was giving Hislop and his defensive colleagues nightmares.

It was no surprise when Beckham eventually found the target, dipping a 25-yard curler beyond Hislop's grasp to double United's lead inside 17 minutes.

Giggs wasted a couple of chances, Roy Keane was close to converting another and van Nistelrooy should have done better than blast straight into Hislop's arms from a Richardson cut-back as Portsmouth groggily tried to steady themselves.

Yet from the ultimate position of weakness, Pompey suddenly grasped a lifeline when, on a rare break, Svetoslav Todorov challenged Roy Carroll for a far post cross which eventually dropped perfectly for Stone to lash into the roof of the net.

Both Paul Merson and Keane found themselves surplus to requirements at the interval but it was the departure of the Portsmouth man which had the bigger impact, his replacement Vincent Pericard's direct style causing mayhem in the home ranks.

Pericard immediately evaded Rio Ferdinand's lunge to shoot at Carroll from an acute angle, then his physical prowess created the space from which Todorov's effort was fatally hindered by an abysmal bobble just as he was about to shoot.

In between, Matthew Taylor flashed a free-kick inches wide before Nigel Quashie wasted a golden chance when he raced free from Gary O'Neil's pass but fired well over with only an advancing Carroll to beat.

Even Ferguson could see the tide was turning against his team, throwing on Scholes for the inexperienced Richardson.

United never looked entirely comfortable after that but at least their midfield had started to tighten, reducing Portsmouth's attacking capability and providing their own platform to counter.

And when Hayden Foxe stupidly attempted to tackle van Nistelrooy as the Dutchman was dribbling aimlessly towards the by-line, the resulting spot-kick was despatched to ease United back into a dominant position.

The home fans still might have endured a frantic last few minutes had Todorov's close range effort not been ruled out for offside but Portsmouth deserved more than the late Scholes chip which flattered the home side.

Teams:

Man Utd: Carroll, Gary Neville, Ferdinand, Blanc, Silvestre (Brown 82), Beckham, Keane (Stewart 45), Phil Neville, Richardson (Scholes 59), van Nistelrooy, Giggs.

Subs Not Used: Ricardo, Forlan.

Goals: van Nistelrooy 5 pen, Beckham 17, van Nistelrooy 81 pen, Scholes 90.

Portsmouth: Hislop, Primus, Foxe, Tavlaridis, Harper, Stone (Burton 89), Diabate (Pericard 45), Quashie, Taylor, Todorov, Merson (O'Neil 45).

Subs Not Used: Kawaguchi, Crowe.

Booked: Diabate, Tavlaridis.

Goals: Stone 39.

Att: 67,222

Ref: M Riley (W Yorkshire).