Benjamin is one of only four Britons competing in Monaco

Benjamin knocked almost two-tenths of a second off his previous mark in running 44.56 seconds behind Tyree Washington.

But world 100m and 200m champion Justin Gatlin finished a shock fourth in the 200m behind Tyson Gay, the man he beat into fourth in Helsinki last month.

Gatlin is expected to be named male athlete of the year on Saturday.

Jamaican Christopher Williams was second and world silver medallist Wallace Spearmon third.

Gay said: "It was a great race. After finishing fourth at the Worlds I was motivated to do my best."

Benjamin, whose second place earned him �11,000, could have won his race had he not relaxed in the final few strides and let Washington edge him out.

"I thought I had it won and it cost me �10,000," he said, although he was more than happy with his big payday as it will help pay the mortgage on a new house he has just bought.

I for one believe he can go sub 44 From I Have your say on Five Live

"Now I'm running so fast I just hope I can continue to do so next season. Judging by that I can run 0.25 seconds quicker."

Wariner pulled up in the final metres of the 400m, clutching his right thigh and hobbling across the line in last place.

Benjamin later revealed he had accidentally made contact with the American as they came down the home straight.

It was Wariner who had looked the likely winner at the mid-race stage before Benjamin and Washington made their moves, but Benjamin said the contact made no difference to the outcome.

He said: "I wasn't worried as I hadn't started my kick, which I did on the final bend."

Britain's other competitors has less success.

Nathan Douglas, who failed to qualify for the World Championships final, finished sixth in his triple jump contest.

Helen Clitheroe finished 10th in her 1500m race with a time of 4min 09.41sec.