Although India imports more than 70 percent of its energy supplies, in recent years, it has opened large areas of territory to oil and gas development. Reliance, which is led by India’s richest man, Mukesh Ambani, has become India’s largest producer of gas because of a rich offshore field in the Bay of Bengal near the state of Andhra Pradesh.

BP would gain access to reserves in India while offering its expertise in deep-sea drilling and its technology, said Christine Tiscareno, an analyst at Standard & Poor’s.

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“It’s a deal of equals,” she said. “There are very few places now that you can find oil, and this is a win-win situation.”

BP’s chief executive, Robert Dudley, is eager to steer his company on a new growth path after it was forced to sell assets to pay for the accident in the Gulf of Mexico. Its shares still trade well below their levels before the rig explosion in the gulf last year.

Mr. Dudley said this month that he wanted BP to be a “safer,” “smaller” and “more agile” company.

Mr. Dudley has sought to reverse a drop in production by increasing investment and lining up 32 projects, including in Russia and Canada, for the next six years.

The deal in the Russian Arctic with Rosneft was generally seen as a coup. It would give BP access to exploration licenses in one of the world’s last unexplored basins. A seal of approval from the Russian government for the deal also guaranteed some safety to operate in what has proved in the past to be a challenging country for BP and other oil majors.

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For international oil companies like BP, forging cooperation agreements with national oil companies is one of the few ways to gain access to new exploration projects and meet growing energy demand, especially in fast-growing economies like India, Russia and China.

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“Virtually all of the growth will come from emerging economies like India,” Carl-Henric Svanberg, BP’s chairman, said during a news conference in London on Monday.

The Reliance deal will require approval from Indian regulators, which could take some time. Indian officials have been slow to approve a deal announced in August by Vedanta Resources, based in London, to buy a controlling stake in Cairn India, an oil-producing subsidiary of Cairn Energy, based in Edinburgh. Officials say they want better terms for Cairn’s Indian partner, the state-owned ONGC, as a condition of approval.

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Mr. Dudley visited India in October and met with the prime minister, Manmohan Singh, and other officials. BP already has a joint venture with Reliance on one exploratory offshore field, and it has a partnership to make solar panels with the Tata Group, a large Indian conglomerate.

In a show of symbolism meant to highlight the long history of relations between Britain and India, Mr. Dudley and Mr. Ambani signed their agreement Monday at 11 Downing Street, the home of George Osborne, the British chancellor of the Exchequer, who witnessed the ceremony.

For Reliance, the deal provides a large hoard of cash and, perhaps more important, oil and gas expertise that it needs as it explores and produces in fields that cover about 104,000 square miles. The output from Reliance’s most productive gas field in the Bay of Bengal has declined in recent months, which has worried analysts and policy makers, who are counting on the field to fuel power plants and fertilizer factories.

In addition to its interests in India, Reliance has been investing in shale gas fields in the United States, including a joint venture with Atlas Energy to drill in the Marcellus Shale.

The Indian government estimates that it has large reserves of oil (8.8 billion barrels) and gas (50.7 trillion cubic feet). That is only a small fraction of the oil and gas reserves of Russia — which has an estimated 1,680 trillion cubic feet of natural gas and 60 billion barrels of oil, according to the United States Energy Information Administration.

But India’s estimated reserves are still significant, as global demand continues to rise. And the country has tapped but a small fraction of its potential supplies. A big chunk of those resources lie deep under the seabed around the Indian peninsula and would be hard to reach safely.

Mr. Ambani said in his company’s assessment, “BP is the best finder of hydrocarbons in deep water.”

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BP said this month that its earnings rose 30 percent in the fourth quarter, to $5.6 billion, from $4.3 billion the period a year earlier, helped by higher oil prices. The company lost $3.7 billion last year, compared with a profit of $16.6 billion in 2009. BP also restored its dividend in the fourth quarter.