LONDON - British Labour leader Keir Starmer said on Sunday his party's "single defining mission" if elected to govern would be to spur economic growth, expressing confidence he could partner with business to achieve that "very quickly".

At the beginning of his opposition party's annual conference in the northern city of Liverpool, probably the last before the next national election, Starmer said his reshaping of Labour was "bang on schedule" and the party was ready to take power.

With a healthy lead over the governing Conservatives before an election expected next year, Starmer said he would use the conference to "set out his stall", offering voters a series of policies that would spur growth.

But the 61-year-old is under pressure to break his fiscally cautious approach to policy, with the head of Labour's biggest donors, the Unite trade union, calling on Starmer to be bolder and return to the left's traditional values.

"We have to create the conditions for the growth in this country ... I am confident that we will get that growth. It is the single defining mission of an incoming Labour Government," Starmer told the BBC.

He said a Labour government would partner with business by providing investors with the "stability" he said had so lacked under the Conservative government which at its party conference last week called off a high-speed rail project.

"Everything hangs off that," he said, adding only growth could fund the renewal of public services.

"We think that this can happen very quickly, within months of a Labour government coming in, we can turn this around and get the investment that we need."

Growth in Britain has been weak by historical standards, and many households have struggled with the soaring cost of living, but official data last month said the country's performance since the start of the COVID-19 pandemic had been much stronger than thought, with faster growth than Germany or France.

Starmer's decision to partner with big business to spur investment and growth has drawn some criticism.

Sharon Graham, general secretary of the Unite trade union, said Labour was "being too timid" with his offer to voters.

"I want Labour to come in and now bat for workers and their communities," she told Sky News. "We've had 13 years of Tories batting for big business and batting for the rich. This is the time for Labour to come in and bat for our side."

(Reporting by Elizabeth Piper, Andrew MacAskill and Alistair Smout; Editing by Elaine Hardcastle)