Prime Minister Narendra Modi on Monday congratulated Liz Truss on being chosen to be the next prime minister of the UK and expressed confidence that under her leadership, the India-UK Comprehensive Strategic Partnership will be further strengthened.

UK foreign secretary Truss on Monday defeated Indian-origin former chancellor Rishi Sunak to be named the winner of the Conservative Party leadership contest and will now go on to formally take charge as British Prime Minister as Boris Johnson's successor.

"Congratulations @trussliz for being chosen to be the next PM of the UK. Confident that under your leadership, the India-UK Comprehensive Strategic Partnership will be further strengthened," Modi said in a tweet.

"Wish you the very best for your new role and responsibilities," he said.

The 47-year-old senior UK Cabinet minister was widely expected to become the third female British Prime Minister after 170,000 online and postal votes cast by Tory members, ending Sunak's historic run as the first member of Parliament of Indian heritage to compete for the top job at 10 Downing Street.

Truss polled 81,326 votes, compared to Sunak's 60,399 in an election with a high turnout of 82.6 per cent, with 654 rejected ballots from a total of 172,437 eligible Tory voters.