Sir Sadiq Khan issued an appeal for businesses to invest in London as Sir Keir Starmer and Narendra Modi signed a “historic” UK-India trade deal.
At the Prime Minister's country residence Chequers, Sir Keir said the agreement marked a "step change" in relations.
Mr Modi said they were "writing a new chapter" in the UK and India's shared history.
The Prime Minister and his Indian counterpart also agreed to ramp up joint efforts to tackle illegal migration and organised crime.
It will see tariffs on an array of British goods reduced from an average of 15% to 3%, with the aim of boosting the £11 billion of imports into the south Asian nation.
Whisky tariffs will be slashed in half, according to the Government, and will fall further over successive years, while other industries including soft drinks, cars and cosmetics are also expected to see cheaper duties.
Sir Sadiq said: “I’m delighted that the Government has signed this historic trade deal with India, delivering a £310m boost for London’s economy and creating opportunities for business in the capital and across the UK.
“On the back of the recent economic deals agreed between the Government and the US and EU, there has never been a better time to invest in the capital.
The deal may also open up opportunities for the more than 1,000 London firms which exported goods worth £1.8 billion to India last year, as well as for other businesses.
There has been some controversy over a “Double Contribution Convention” in the agreement which will mean Indian business people and other professionals seconded to the UK will not pay National Insurance contributions into both countries’ social security system for three years, and vice versa for British staff transferred to India.
Such a system already exists between Britain and the US, Canada, the EU, Japan, and dozens more countries.
Ahead of the official signing of the deal, Business Secretary Jonathan Reynolds said: “It’s a historic day and it’s a great trade deal for Britain.”
On the NICs issue, he added: “It’s completely false to say any British worker is undercut by this deal.
“The specifics of this are that a person on a temporary secondment from an Indian company to the UK or UK company to India pays into their own social security systems for a short-period of time.”
The deal is expected to result in 2,200 jobs across the country and £6 billion investment by British and Indian businesses, according to the UK government.
Britain and India are also bolstering co-operation on tackling corruption, fraud, organised crime and illegal migration, by sharing criminal records and other intelligence.
Shadow business secretary Andrew Griffith said: “Any gains from this trade deal will be blown out of the water by Angela Rayner’s union charter, stifling business with red tape, the jobs tax and, come autumn, Rachel Reeves’ inevitable tax hikes.”
Sir Keir is facing calls to raise with Mr Modi the case of Jagtar Singh Johal, a British citizen who has been detained in India since 2017.
The Scottish Sikh is accused of being a member of the Khalistan Liberation Force, which is banned as a terror group in India.
His family say he is being arbitrarily detained.
