Johnson travelled to India as he faces criticism at home and the embarrassing prospect of a probe into whether he lied to parliament over the lockdown-breaking "Partygate" scandal.

India is part of the Quad grouping with the United States, Japan and Australia that is seen as a bulwark against an increasingly assertive China.

But New Delhi also has a long Cold War history of cooperation with Moscow, still its biggest military supplier, and has refused to condemn Russia for its invasion of Ukraine.

"The threats of autocratic coercion have grown even further," Johnson said alongside his Indian counterpart Narendra Modi, in an apparent reference to Beijing.

"And it's therefore vital that we deepen our cooperation, including our shared interest in keeping the Indo-Pacific open and free."

The new partnership was "a decades-long commitment", he added, hailing the relationship between "one of the oldest democracies, and India, certainly the largest democracy".

But relations between Britain and India have long been coloured by the legacy of colonial rule -- when London saw the world's second-most populous nation as the jewel in the crown of its empire but hundreds of millions of Indians chafed under its authority.

Modi's Hindu nationalist government regularly emphasises the independence struggle as a vital component of India's national identity.

It has built giant statues of key independence leaders and created a museum to one of them in New Delhi's world heritage-listed Red Fort.

It was "historic" that Johnson's visit to India came in the 75th year of its independence, Modi said.

"We discussed several regional and international developments and stressed a free, open, inclusive and rule-based order in the Indo-Pacific," he added.

Exact details of the security partnership were not immediately available.

But Johnson said the two had agreed to work together in defence procurement "to meet threats across land, sea and air, space and cyber, including partnering on new fighter jet technology, maritime technologies to detect and respond to threats in the oceans".

New Delhi has long sought to bolster its domestic military manufacturing capacity, partly to reduce its dependence on Moscow and partly as a contribution to its "Make in India" campaign.

- Diplomatic visitors -

A stream of diplomatic visitors have been making their way to New Delhi in recent weeks as a US-led alliance seeks to lure India away from Moscow, promising enhanced security, defence and energy cooperation.

Modi had telephone talks with US President Joe Biden last week, while European Commission chief Ursula von der Leyen is due to arrive on Sunday.

For their part, Russian foreign minister Sergei Lavrov and his Chinese counterpart Wang Yi have also visited in recent weeks.

India has urged cessation of violence while refusing to explicitly condemn Russia, and has abstained on Western-backed United Nations resolutions censuring Moscow.

It argues it needs to ensure its security and energy needs are not compromised and has bristled at criticism of its purchases of Russian oil, pointing to Europe's far larger buys.

Johnson began his maiden India visit as prime minister on Thursday in Modi's home state Gujarat, where he announced new investments worth pound1 billion ($1.3 billion) that he said will help create 11,000 jobs across the UK.

Johnson said the two countries were on the path to finalise a post-Brexit free trade deal by October that has been held up by New Delhi's desire to secure more visas for Indians to work or study in Britain.

"We are making full use of the freedom that we now have to reach a Free Trade Agreement, a deal where you can lift those tariffs," he said.