David Cameron has pledged to create a "modern, dynamic partnership" with India after welcoming Prime Minister Narendra Modi to Downing Street.
At a joint press conference, the Prime Minister admitted that links between the countries had been "imprisoned by the past" up to now.
But as the leaders unveiled £9 billion of business deals, he insisted it was time to "set this relationship free".
Mr Cameron also played down criticism of Mr Modi's record as chief minister of Gujarat - including over 2002 riots where more than a thousand Muslims were said to have died - pointing out that he had an "enormous mandate" from the Indian people.
With noisy protests taking place outside Downing Street, Mr Modi - the first Indian premier to visit Britain in nearly a decade - was brought into Number 10 via a back route instead of coming through the front gates.
The two men shook hands on the steps, before holding talks with their ministers around the Cabinet table.
At a press conference in the opulent surroundings of the Foreign Office's Locarno Room later, Mr Cameron hailed a "real opportunity to open a new chapter in the relationship between our two countries" and develop "a more ambitious modern partnership" politically as well as on the economy and defence.
British and Indian companies would be announcing new collaborations worth a total of more than £9 billion during the three-day visit, he said.
Mr Cameron said the UK should be India's "number one partner" in raising finance for Mr Modi's plan to create 100 "smart cities" across the country, with the City of London establishing itself as the world's centre for trading in India's rupee currency.
This process was beginning with the issuing of £1 billion worth of bonds in London, including the first-ever government-backed rupee-denominated bond to be issued internationally, said Mr Cameron.
Amravati, Indore and Pune are three of the cities taking part in Mr Modi's smart development programme, and Mr Cameron said he hoped British firms would get a share of the planning, design and construction work involved. Indian and British scientists will work together in a new £10 million research collaboration into low-cost, low-carbon energy sources for the smart cities.
Britain is already the largest investor in India among G20 countries while India invests more in the UK than it does in the rest of the EU combined.
"I think it is time to set this relationship free from these misconceptions and from the past," Mr Cameron said.
Mr Modi flatly rejected criticism of his record in Gujarat and allegations of reduction in civil liberties since he came to power.
"India is a land of Gandhi and therefore there is something that is deeply entrenched in our culture, our traditions, which is that we are not accepting anything that is having to do with intolerance," he said, speaking through an interpreter.
"Any incident that happens is a serious incident and we do not tolerate these incidents at all."
He denied that he had been blocked from coming to Britain in 2002, saying he wanted to "correct this wrong perception".
Asked whether he was worried about controversy surrounding Mr Modi, Mr Cameron said: "I am pleased to welcome PM Modi here. He comes with an enormous mandate from the people of India who made him prime minister with a record and historic majority.
"As for what happened in the past there were legal proceedings, there were also ... representations from the British government at the time.
"We are now discussing the future partnership between Britain and India.
"Both of us are backed by our countries for this parliament to work together and strengthen the partnership that we have."