Celtic took another step towards retaining the Scottish Premier League title with a convincing win at Dundee United.

John Hartson went some way to proving he is worth a starting place by grabbing the first, and then Stilian Petrov increased the home side's advantage.

Alan Thompson put the game beyond doubt when he curled a free-kick home shortly after the re-start, and striker Henrik Larsson sealed the win 19 minutes from time.

Chris Sutton's injury gave Hartson the opportunity he had been waiting for as he looks to show he can be just as effective in front of goal as the former Chelsea and Blackburn hitman.

And the £6million man wasted no time in proving his worth at Tannadice, putting goalkeeper Paul Gallacher under pressure as early as the second minute when his shot forced Gallacher into an impressive save.

The big Welshman struck again after four minutes and this time he was more effective as he pounced on a Jackie McNamara through ball before slotting home into the bottom left-hand corner of the net.

Hartson was proving to be a major one-man threat and he had another couple of chances in the space of two minutes.

He just failed to connect with a Bobo Balde header in front of goal after eight minutes and had the ball in the back of the net again after 10 minutes.

But referee John Rowbotham ruled that the former Wimbledon striker had pushed David Partridge and the goal was disallowed.

It was no surprise, however, that Hartson was the man responsible when Celtic doubled their lead after 26 minutes.

Henrik Larsson and McNamara exchanged a cheeky one-two before McNamara squared the ball for Hartson to tap home from a few yards out.

Only the heroics of the Dundee United goalkeeper prevented Celtic from piling further misery on the Tannadice side.

He produced a super save to deny Larsson's chip after 28 minutes and was called into action again a minute later.

This time the Swede connected awkwardly with a Thompson corner, the ball bounced off his body and only another fine save from Gallacher stopped the ball from tumbling over the line.

The home side's chances had so far proved to be few and far between but they tried to put Celtic under pressure five minutes before the break when Jim Paterson pinpointed Thompson in the box but the danger was cleared before the United forward could connect.

Unfortunately for Dundee United, Celtic started the second half in the same rampant form that had seen them net twice in the opening 45 minutes.

A Jim Lauchlan tackle on McNamara on the edge of the box proved costly for the Tangerines as Celtic took the opportunity to extend their lead.

Thompson stepped up for the set-piece and left Gallacher with no chance as he curled the ball into the net.

The game took another turn for the worse for the Tannadice side when manager Alex Smith was forced to replace the injured Paterson. He appeared to be caught in the face by McNamara while trying to win a header and had to be stretchered off and replaced by Jim Hamilton after 51 minutes.

Youngster Stephen Crainey had been drafted into the side in place of the suspended Joos Valgaeren but his big opportunity to impress ended in misery when he, too, was stretchered off.

The young defender was the victim of a harsh challenge by Hamilton and he was swapped for Olivier Tebily after 62 minutes.

Despite the upset, Celtic continued to push forward and had another chance just a minute later when Neil Lennon curled the ball into the box only for Larsson to nod well wide.

The strike partnership of Hartson and Larsson was proving to be fruitful as Celtic put the game beyond doubt after 71 minutes.

Hartson played the ball through to the Swede and he smacked the Hoop's fourth goal of the game in off the post.

Dundee United midfielder Charlie Miller was lucky to escape the first booking of the game for a nasty challenge on Petrov after 74 minutes but just seconds later the referee opted to caution Thompson for dissent.

Rowbotham was reaching for the yellow card again after 82 minutes as the game became more heated - this time Partidge's name went in the book for a foul on Larsson.

Despite his earlier reluctance, the official eventually cautioned Miller two minutes later for a challenge on Bobby Petta.

Even as the game entered stoppage time, Celtic were still searching for another goal and Thompson could have claimed his second of the day but nodded wide from a Hartson cross.

Teams

Dundee Utd: Gallacher, Cocozza, Lauchlan, McCracken, Partridge, Easton, Venetis (O'Brien 79), Hannah, Miller, Paterson (Hamilton 53), Thompson (Lilley 70).

Subs Not Used: Combe, Wright.

Booked: Partridge, Miller.

Celtic: Douglas, Balde, Lennon (Petta 72), Mjallby, McNamara, Petrov, Lambert (Maloney 77), Thompson, Crainey (Tebily 62), Larsson, Hartson.

Subs Not Used: Gould, Moravcik.

Booked: Thompson.

Goals: Hartson 4, Petrov 27, Thompson 47, Larsson 71.

Att: 12,165

Ref: J Rowbotham (Scotland).