LONDON (Reuters) - British Prime Minister Rishi Sunak will unveil a major package for Ukraine, including $3 billion of additional guarantees to unlock World Bank lending, on the first day of a summit aimed at spurring efforts to rebuild the country on Wednesday.

At the beginning of the two-day Ukraine recovery conference in London, Sunak will outline a package which will also include 240 million pounds ($306 million) of bilateral assistance and an expansion of British International Investment in Ukraine.

Britain has been one of the main backers of Ukraine since Russia launched its full-scale invasion in February last year, and Sunak hopes the conference will cement London's standing while encouraging the private sector to do more to help rebuild.

"So, together with our allies we will maintain our support for Ukraine's defence and for the counter offensive, and we'll stand with Ukraine for as long as it takes as they continue to win this war," Sunak will tell the conference, according to excerpts of his speech released by his office.

"I'm proud that today we're announcing a multi-year commitment to support Ukraine's economy, and over the next three years, we will provide loan guarantees worth $3 billion."

Ajay Banga, president of the World Bank Group, said the guarantees would allow it to continue to help "people rebuild their lives after devastation".

The United States is also expected to set out "a new, robust" assistance package for Ukraine on Wednesday.

After more than a year of war in Ukraine, Sunak hopes the conference will encourage the private sector to use its resources to help speed Ukraine's reconstruction, but officials will also have to wrestle with the issue of offering some kind of insurance against war damage and destruction.

His office said he would also launch the London Conference Framework for War Risk Insurance at the summit and that some major companies had already signed up to the so-called Ukraine Business Compact, a statement of support for Ukraine's recovery.

Ukraine is seeking up to $40 billion to fund the first part of a "Green Marshall Plan" to rebuild its economy, including developing a coal-free steel industry, a senior Ukrainian official said before the conference.

The total bill will be huge, with Ukraine, the World Bank, the European Commission and the United Nations estimating in March that the cost was at $411 billion for the first year of the war. It could easily reach more than $1 trillion.

($1 = 0.7851 pounds)