Watford, to their credit, provided stiffer opposition than Roma, but class will out in the end, and Manchester United are in the FA Cup Final and on their way to what could be another historic Treble. When Wayne Rooney scored the first of his two goals after only seven minutes it looked like it could be another United landslide, but Watford fought back gamely to equalise through Hameur Bouazza before Rooney, Cristiano Ronaldo and substitue Kieran Richardson rattled in three to settle the issue.

Safely through to yet another final - a record 18th - United’s only concern will be the loss of Rio Ferdinand, who went off in the first half with a groin strain. Already without Nemanja Vidic for the rest of the season with a broken collarbone, and with Mikael Silvestre also hors de combat, they can ill afford the loss of another centre-half.

Not all the tickets had been sold, Watford unable to dispose of their full allocation, but the Holte End was full to bursting with their supporters, and if the numbers were slightly down on expectation, there was no faulting the atmosphere. The class divide between the two teams was such that it seemed more like the little and large of the third round than the semi-finals, but Watford were commendably competitive. United, their resources so much greater, were able to recall Paul Scholes, suspended for the annihilation of Roma in midweek, in place of Darren Fletcher.

Also back was Patrice Evra, to the exclusion of John O’Shea. Evra was at right-back, instead of his customary station on the left, enabling Wes Brown to partner Ferdinand in central defence. It was a double act that lasted only until the 40th minute, when Ferdinand limped off and, with Fletcher on at right-back as substitute, Sir Alex Ferguson switched Gabriel Heinze into the middle.

Watford, bottom of the table and relegation-bound, had their disadvantage exacerbated by the inability to play Ben Foster, the England goalkeeper, who is on loan from United. The unsung Richard Lee deputised and distinguished himself with the save of the match, from Rooney. There was a surprise return among Watford’s substitutes for the striker who shot them to promotion last season, Marlon King. The Jamaican international had been expected to miss the rest of the season, and his fan club celebrated as if they had won the Cup when their hero’s name was announced shortly before kick-off. They would have preferred to have him in the starting lineup, of course.

Aidy Boothroyd, the Watford manager, said in these columns last week that all his players needed to give 10 out of 10 performances, and the luck to go their way, if they were to cause an upset. It was asking a hell of a lot. Too much. United were ahead early, and after that there was likely to be only one outcome. Michael Carrick’s pass threw the Watford defence into the state of disarray that was to become the norm, Ryan Giggs stepped over the ball and Alan Smith’s lurking presence wrong-footed Adrian Mariappa for Rooney, coming in from his starting position on the left, to step coolly inside him before letting fly with a shot from the 18-yard line that flew high past Lee’s helpless right hand.

Behind so soon, Watford were in danger of going the way of Roma. Running and transferring the ball swiftly, and using it with perceptive precision, United’s kaleidoscopic attack regularly threatened to add to their lead, forcing their ill-equipped opponents to dig deep to avoid embarrassment. To their credit, they applied theselves in the best tradition of underdogs everywhere, and Bouazza, their leading scorer, raised their morale with a 25-yard shot, from right to left, that was too close for Edwin Van der Sar’s liking.

The big Dutchman was even less impressed when Damien Francis clattered into him, drawing blood from his nose.

After receiving treatment, the goalkeeper was still shaking his head to clear his senses when a throw-in from the left from Jordan Stewart found its way to Tommy Smith, who helped it into the middle for Bouazza, in a central position, 10 yards out, to score with a spectacular overhead volley which went in via the underside of the crossbar. The situation cried out for Watford to consolidate their new-found equality, but they couldn’t. Instead, United were ahead again within a minute when Rooney, after exchanging passes with Smith, drove to near the byline on the right before cutting the ball back for Ronaldo to bundle it home at nudging range, past Adrian Mariappa’s desperate lunging intervention. The man of the moment was at it again, pushing his candidacy for Footballer of the Year. United would have had a third just before half-time but for the outstanding reaching save with which Lee kept out Rooney’s close range shot.

Nothing if not game, Watford pushed forward in search of a second equaliser, and were not far from it when the United defence failed to clear a long throw-in and Bouazza was only a foot wide from six yards. The French-Algerian was soon rampaging back, running past Heinze, who was grateful for Evra’s timely tackle on the edge of the penalty area. United need to draw the Hornet’s sting and sought to do so by slowing play down with a prolonged session of keep-ball.

This culminated in Smith setting up their third goal, crossing from the right for Rooney to supply a characteristically emphatic finish at the far post. United had the two-goal cushion they wanted and, with half-an hour still to play, it was game over.

Star man: Wayne Rooney (Manchester United)

Player ratings. Watford: Lee 6, Mariappa 6, Demerit 6, Carlisle 6, Stewart 5, Chambers 5 (Kabba 70min), Francis 7, Mahon 5, Smith 6, Priskin 5 (King 78min), Bouazza 7

Manchester United: Van Der Sar 6, Evra 5, Ferdinand 5 (Fletcher 40min, 6), Brown 6, Heinze 6, Ronaldo 7 (Richardson 77min), Scholes 7, Carrick 7, Giggs 6 (Solskjaer 83min), Rooney 8, Smith 6

Scorers: Watford: Bouazza 26 Man Utd: Rooney 7, 66, Ronaldo 28

Yellow cards: Man Utd: Rooney, Carrick

Referee: H Webb

Attendance:37,425