Portsmouth still have a healthy 10-point cushion over the third-placed Blades, but Neil Warnock’s team have two games in hand over the leaders and one over Leicester.

United’s remarkable season has them targeting promotion and Worthington Cup glory, and they were never intimidated at Fratton Park.

Having taken the lead through Peter Ndlovu after 24 minutes, United survived two second-half scares when Vincent Pericard and Paul Merson hit the woodwork.

Gary O’Neil looked to have rescued a point for Portsmouth when he netted with 12 minutes to go, but Brown’s cool finish produced the Blades’ eighth away win of the season.

Ndlovu, who had spurned an early chance when he failed to get a meaningful touch from close range, powered the visitors in front.

The Zimbabwean made the most of skilful approach work from Michael Tonge to fire in a shot beyond Shaka Hislop into the bottom corner of the net.

United created the better chances in the first half and should have extended their lead six minutes before the break.

This time Ndlovu turned provider with an excellent through ball, pitting Brown against defender Linvoy Primus. He mis-timed his challenge to leave Brown with only Hislop to beat but the goalkeeper got a slight touch with his leg, deflecting the effort onto the crossbar.

Portsmouth had produced good approach work but lacked the finish in the first half, Steve Stone seeing an 18-yard effort blocked and Svetoslav Todorov firing a weak shot from close range.

But they found their shooting boots in the second half, with Pericard coming agonisingly close to an equaliser.

He took on the United defence eight minutes after the break to work himself into a shooting position, only to fire across Paddy Kenny and against the foot of the post.

Paul Merson was equally unlucky on 74 minutes when, after Shaun Murphy’s foul on O’Neil 25 yards out, he struck his free-kick over the wall but against the crossbar.

Three minutes later Portsmouth earned reward for their determined second-half display when a move involving Merson saw Todorov slip the ball through for O’Neil to poke his shot over the advancing Kenny.

For a time, Portsmouth looked the more likely winners and they had a penalty appeal when Pericard’s header struck the raised arms of Shaun Murphy.

But United struck back with the winning goal in the 88th minute. Ndlovu fed the ball through after a defensive mistake and Brown kept cool to slot the ball low past Hislop’s dive.