By Michael Holden

LONDON (Reuters) - London's High Court will deliver its verdict on Thursday on whether British lawmakers, rather than the government, must trigger the formal process of leaving the European Union, a decision that could affect the circumstances of Britain's exit.

The court heard a challenge last month from campaigners who argued Prime Minister Theresa May and her ministers do not have the authority to invoke Article 50 of the EU Lisbon Treaty, the mechanism by which a nation can leave the bloc, without the explicit backing of parliament.

If the challengers win their legal fight, lawmakers might have to vote on whether and when Article 50 should be triggered, which could lead to delays or even, in theory, block Brexit altogether.

May has said she will invoke Article 50 by the end of March next year, starting a two-year divorce procedure from the bloc. Sterling plunged in the wake of her announcement, shedding about 5 percent against the dollar and hitting its lowest levels in over three decades.

The court case is being closely monitored by market players who believe the more parliament is involved, the greater the chance there is of a "soft Brexit", where Britain stays in or remains close to the EU single market.

The High Court said on Wednesday that the judgment of the Lord Chief Justice John Thomas and two other senior judges would be handed down on Thursday at 6.00 a.m. ET.

The losing side is expected to appeal to the Supreme Court, the UK's highest judicial body, which is likely to hear the case in December.

During the three-day hearing last month, the Attorney General, the government's top lawyer accused the challengers of trying to invalidate the result of June's referendum in which Britons voted by 52 to 48 percent in favor of leaving the EU.

He said it was established constitutional convention for the executive to make or withdraw from international treaties using the ancient power of "royal prerogative", and told the court that once invoked, the process to leave the EU was irrevocable, a contrary view to that held by Donald Tusk, the European Council president who will oversee Brexit talks.

The challengers, whose lead claimant is investment manager Gina Miller, argued that rights granted to Britons under the European Communities Act of 1972 cannot be taken away by the executive without the approval of parliament, the sovereign body under Britain's constitution.

If their court challenge is ultimately successful, the government could be forced to pass legislation authorizing Article 50 to be triggered, requiring a vote by MPs, the majority of whom voted for Britain to remain in the EU although most are likely to endorse Brexit now.

(Editing by Stephen Addison)