On his first visit to New Delhi as Britain's prime minister, Boris Johnson will discuss with India's leader Narendra Modi ways to boost trade and security ties.
The South Asian country currently buys more than half its military hardware from Russia.
"This is a very auspicious moment in a relationship and friendship between two democracies," Johnson said after Modi welcomed him at the presidential palace ahead of the talks on the final day of a two-day visit.
"I don't think the relationship has ever been as good."
The effort follows an attempt last month by the United States to move India away from Russia, with its own offer of more defence and energy sales, after President Joe Biden called India "somewhat shaky" in acting against Moscow.
Britain hopes its offer of closer security ties with the West will encourage India to distance itself from Russia following the invasion of Ukraine.
India abstained from a United Nations vote condemning the invasion and has not imposed sanctions on Moscow, taking a stance that sparked off a recent flurry of diplomatic activity, including visits by the foreign ministers of Russia and China.
Britain said it plans to support India's long-held goal of building its own fighter jets, to reduce expensive imports of military equipment.
India currently has a mix of Russian, British, and French fighter jets.
Former colonial ruler Britain will issue a so-called open general export licence to India to shorten delivery times for defence items.
Only the European Union and the United States currently have such licences, an official said.
The offer of closer security ties would also bring more joint military exercises and officer exchanges.
While Britain's offer of greater co-operation was welcome, India will remain reliant on Russian military equipment, partly because it is cheaper, Mohan Guruswamy, a director of the Centre for Policy Alternatives in New Delhi, said.
"Russia's offer to India is always that 'Our cupboard is open', (but) Britain doesn't offer India everything it wants and what it does make available is usually more expensive," he said.
Johnson and Modi will also discuss trade as they push for a deal well ahead of general elections in both nations in 2024.
British trade with India in 2019 was worth Stg 23 billion ($A41 billion), British data shows - an amount dwarfed by trade with neighbouring Ireland, a nation of about five million people.
But with the world's second-biggest population of nearly 1.4 billion, India offers a vast pool of possible customers.
Any trade deal is likely to depend on relaxing rules and cutting fees for Indian students and professionals going to Britain.
Speaking during the journey to India, Johnson signalled he was ready to be more accommodating on the issue, adding Britain faced a massive shortage of hundreds of thousands of workers, particularly in sectors such as information technology.
