Goals from Ken Sema, Nathaniel Chalobah and Andre Gray strengthened Watford's Championship promotion charge as they saw off struggling Birmingham.

Sema netted his second goal in as many games to give the Hornets an early lead before Chalobah powered in a second-half header.

Substitute Gray wrapped up the home side's victory with a late third, just seconds after coming on.

It was Watford's fifth straight win and their ninth in 10 games, although they are still eight points behind leaders Norwich.

Birmingham, who tasted defeat for the first time under new boss Lee Bowyer, remain 21st, three points above the bottom three.

Watford unlocked the Blues defence in the fourth minute as Joao Pedro's skidding drive was parried by Neil Etheridge and the rebound fell perfectly for Sema to tuck away.

Sema then turned creator, delivering a corner that Adam Masina put wide at the far post and also setting up Isaac Success - but the Nigerian forward, making his first league start in two years after an injury lay-off - failed to take advantage.

Birmingham held their own for a while and Jeremie Bela almost pulled them level midway through the first half, only to be denied by Chalobah's courageous block.

But the Hornets captain's next significant contribution came at the other end, 10 minutes after the interval, as he peeled away at the far post to head home Dan Gosling's corner and make it 2-0.

Lukas Jutkiewicz went close to reducing the deficit before Gray, brought on to give Sema a rest, sealed Watford's triumph nine minutes from time.

Birmingham conceded possession on the halfway line and the Hornets striker raced clear, clipping beyond Etheridge for his fourth goal of the campaign.

Watford head coach Xisco Munoz told BBC Three Counties Radio:

"It's important to celebrate today, but we have a big job in the future and we need to work very hard for the last eight games.

"We have another clean sheet, which is important and the team gave 100 per cent. We have eight important games in the future and I don't want to lose the focus.

"I think I have very good luck because I have very good players and it doesn't matter who plays. We have a very good spirit and the players on the pitch give their best for the squad, that's amazing for me."

Birmingham City boss Lee Bowyer told BBC Radio WM:

"They were more clinical than us and goals win games, unfortunately. Let's be honest, they're a Premier League side that's come into the Championship and they're going to go back up, that's the reality.

"We could have done a bit better with the first two, especially the set-piece, but I think we caused them a lot of problems. We moved the ball well at times and I felt we were the more likely to score.

"Even when we were two-down I still felt 'you know what, if we get one, we might just nick another'. On another day, I think Jukey (Jutkiewicz) scores a couple, but Watford defended really well.

"The players deserve a lot of credit because I've been there for three or four days and we haven't had much time on the training pitch at all."