Reigning Olympic 200 metres queen Veronica Campbell-Brown conceded Wednesday it will be difficult to successfully defend her crown at the London Olympics.

The 30-year-old has not been enjoying the best of seasons, suffering a string of defeats this year which have hinted at chinks in her armour.

She told journalists here that she had worked hard on her preparation for the Olympics, however, and would be giving the title defence her best shot.

"I put in all the work and I know it will be absolutely difficult for me to claim victory here in the 200m but anything can happen," she said.

"I prepared well and so I just have to keep my mental focus right and make sure that I get my race together. It will come down to execution and mental focus and I will go out there and I will give it my all and we will see what happens."

Campbell-Brown has run a season-best 10.82 seconds in the 100 metres for second behind Olympic champion and fellow Jamaican Shelly-Ann Fraser-Pryce (10.70) at the Jamaica trials.

She was again beaten in the 200m at the trials, running 22.42 behind Fraser-Pryce and Sherone Simpson.

But the most notable of her defeats came in Lucerne on July 17, when she ran 22.70 for second behind American Charonda Williams (22.52).

Campbell-Brown, who will run both 100m and 200m races at the Olympics, agreed it had not been her best year but said she was remaining optimistic.

"I will not dwell on those (bad races) and look on those as something negative… I learned from those races and I am going into my competition here very confident," she said.

"I am going to go out there and give it my best and only God knows what the result will be. I have trained hard this season and not because I had a few races that did not went too well, I am not going to let that deter my confidence or allow it to let me feel weak or anything."

Campbell-Brown won back-to-back 200m titles in Athens and Beijing, on both occasions beating American Allyson Felix, a three-time world champion over the distance.

However, she won her first 200m world title last year in Deagu.