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Former Starbucks chief executive, Howard Schultz, has denied union busting in his most pointed public comments yet on the ongoing dispute between labour organisers and the coffee giant in the US.

Labour officials have repeatedly found that the firm has broken federal laws in its response, including wrongfully firing workers.

Mr Schultz, who stepped down as boss this month, was called to appear before Congress to answer for the findings.

He "unequivocally" denied the claims.

"Those are allegations and Starbucks has not broken the law," Mr Schultz said, adding that the disputes were still being litigated.

While Republicans largely held back from criticism, Democrats challenged Mr Schultz's assertions that the company was respecting worker rights.

"It is akin to someone ticketed for speeding 100 times saying I've never violated the law because every single time the cop got it wrong," said Senator Chris Murphy. "That would not be a believable contention."

The two-hour hearing in Washington comes more than a year after baristas at a Starbucks coffee shop in Buffalo voted to form a union, a step since taken by more than 270 stores. Starbucks Workers United says it now represents roughly 7,000 people.

Starbucks has vigorously opposed the campaign, which has sparked a debate about inequality and the workings of capitalism and threatened to tarnish the firm's reputation as a progressive employer.

Mr Schultz, who led the company for years and returned as chief executive last year, defended its practices, saying the union represented a tiny fraction of the company's more than 9,000 US stores and the firm had a right to share its preference that it have a "direct relationship" with its staff.

He said the company offered industry-leading benefits, including average hourly wages above $17, access to health insurance, college tuition support and stock grants.

"It's unprecedented and it's why we don't need a union," he said. Mr Schultz, who remains a Starbucks board member, later challenged lawmakers: "Are you aware of a union contract that has those benefits?"

At times, the hearing grew testy as Mr Schultz, who grew up in subsidised housing and has flirted with running for president, bristled at characterisations that he was a heartless "billionaire".

"I came from nothing," he said, adding that he had earned his wealth, which Forbes estimates at roughly $3.7bn. "It's unfair."

Democrats pressed Mr Schultz to explain why union members and the company have yet to agree contracts and refused to extend raises and other perks granted to non-union staff last year.

He said the company had met dozens of times and was willing to negotiate with workers, but did not want to bargain on video calls, worried it could not confirm who was participating.

He said the company did not have to extend benefits while bargaining and did not want to negotiate a contract "piecemeal".

Republicans focused their questions on whether labour officials were biased against the firm, describing the hearing, which was convened by Senator Bernie Sanders, a "smear campaign".

"I am not here to defend Starbucks, I have my own questions about the alleged misconduct and the law should be followed and upheld but let's not kid ourselves - this is not a fair and impartial hearing," said Republican Senator Bill Cassidy of Louisiana.

Mr Schultz's testimony was followed by a panel, which included workers involved in the campaign.