Karthikeyan was dropped by Midland at the end of last season

Karthikeyan, who impressed the team in a test last month, will have a support role to reserve driver Alexander Wurz.

Karthikeyan, 28, was dropped by rival team Midland, formerly Jordan, after driving for them last season.

Team owner Sir Frank Williams said: "Having a year's GP racing and testing to his credit is advantageous for us."

It has taken some while to resolve what I would be doing for 2006, but joining the team was the best option for me Narain Karthikeyan

Karthikeyan's best finish in 2005 was fourth in the US Grand Prix, where only six cars took part following a row over tyres.

But Williams bosses were pleased with his performance during a test at Jerez in southern Spain late last year.

"Narain's brief period in the car in December, during which he provided valuable feedback, made a very distinct impression on our engineers," added Williams.

Wurz secures Williams test role

Although the move represents a significant drop down the F1 pecking order, the former Midland driver insists he is happy.

"It has taken some while to resolve what I would be doing for 2006, but having tested for Williams late last year, joining the team was the best option for me and I am delighted that we can finally confirm this," said Karthikeyan.