* Repeal Bill, a key Brexit law, expected to be published
* Legislation will test May's authority after election flop
* Firms eagerly awaiting bill to assess business impact
By William James
LONDON, July 13 (Reuters) - Britain will publish on Thursday the legislation that will sever its political and legal ties to the European Union, beginning what is likely to be a divisive debate that will test Prime Minister Theresa May's ability to lead the country.
The Repeal Bill is central to the government's plan to exit the EU in 2019, providing the mechanism for decades of EU law to be turned into British law, and to enact Brexit by repealing the 1972 legislation that made Britain a member.
Its passage through parliament could make or break May's future as prime minister. The election she called last month cost her an outright parliamentary majority and reopened the debate on the nature of Britain's EU exit.
Within May's Conservative Party, pro-Brexit lawmakers are fiercely defensive of her plan for a clean break with the EU. Pro-Europeans are looking to extract concessions that soften the divorce terms.
Rebellion by either side could derail the legislation and test May's ability to negotiate a compromise or find support from opposition parties. If she fails, her position could swiftly become untenable.
"It is one of the most significant pieces of legislation that has ever passed through Parliament and is a major milestone in the process of our withdrawal from the European Union," Brexit minister David Davis said in a statement
The publication of the bill is the first step in a long legislative process, with no formal debate in parliament expected on Thursday. It will be closely examined to see how the government plans to carry out the difficult and time-consuming technical exercise of transposing EU law.
Lawmakers have expressed concern that the sheer volume of work created by the shift could limit parliament's ability to scrutinise it effectively and ensure the government is not introducing policy change by the back door.
The bill will also face scrutiny from British companies, many of which have spent the year since Britons voted to leave the EU trying to figure out how the change will affect their business.
A successful shift to British law is crucial for many industries that depend on EU-wide regulation.
Davis said the bill would enable Britain to exit the EU with "maximum certainty, continuity and control."
But the financial industry, a major component of Britain's $2.5 trillion economy, is already warning that the government has left it too late to convince them it can strike a deal to soften the impact of Brexit before they start shifting jobs from London.
May's attempts to build broad political support for the Brexit plan she set out in January, which involves leaving the EU's single market and prioritising immigration control, have so far fallen on deaf ears.
The opposition Labour Party Leader Jeremy Corbyn has committed to Brexit. But, emboldened by a surge in public support for his leftist agenda he has set out his own parallel plan for Brexit, hoping that May's government will fall. (Editing by Larry King)