By Estelle Shirbon and William James

LONDON (Reuters) - Prime Minister Theresa May will use a major speech on Brexit next week to call on Britons to reject the acrimony of last year's referendum and unite around the vision of a Britain more open to the world, her office said on Sunday.

May intends to kick off the formal process of negotiating the terms of Britain's exit from the European Union by the end of March, but has given little away about what deal she will be seeking, frustrating some investors, businesses and lawmakers.

She is due to make a speech in London on Tuesday before an audience including foreign diplomats as well as Britain's own Brexit negotiating team and other senior officials, May's Downing Street office said in a statement.

It said she would stress the need for Britons, who voted for Brexit by 52 to 48 percent in last June's referendum after a deeply divisive campaign, to unite around common goals such as protecting and enhancing workers' rights.

"Now we need to put an end to the division and the language associated with it – 'Leaver' and 'Remainer' and all the accompanying insults – and unite to make a success of Brexit and build a truly global Britain," May is expected to say.

Her comments came as finance minister Philip Hammond said Britain could change its economic model to regain competitiveness if it were to leave the EU without an agreement on market access.

Those comments, from an interview with German newspaper Welt am Sonntag, were interpreted as a warning that Britain could use its corporate tax as a form of leverage in Brexit negotiations.

"He appears to be making a sort of threat to the European community," said Jeremy Corbyn, leader of the opposition Labour Party. "It seems to me a recipe for some kind of trade war with Europe in the future."

Nevertheless, Corbyn said he would not block the triggering of 'Article 50' - the legal process of leaving the EU. An upcoming legal ruling could insist that May gets parliamentary approval for her plan to trigger Article 50 by the end of March.

"CLEAN AND HARD BREXIT"?

The extracts of May's speech did not say whether she would reveal her stance on one of the key questions: whether she will try and keep Britain within the European single market or customs union or, if not, what level of access she will aim for.

The Sunday Times and other newspapers said the speech would signal that Britain was seeking a "clean and hard" Brexit, which would involve leaving the single market and the customs union.

The report cited an unnamed source in May's office who said her words were likely to cause a market correction. A spokesman from May's office told Reuters the reports were speculation and did not comment further.

The problem for Britain is that the EU is likely to insist on freedom of movement for EU citizens in return for full access to the single market, while many of those who voted for Brexit did so precisely in order to be able to restrict immigration.

May's speech will be closely watched by financial markets for information on which of these divergent goals she will prioritize.

After she said in a TV interview a week ago that post-Brexit Britain would not be able to keep "bits" of its EU membership, the pound fell sharply as the comment was interpreted as signaling a clean break from the single market.

In a separate Sunday Times article, Brexit minister David Davis wrote that Britain would consider ways to extend or smooth the exit process to provide certainty for businesses.

"If it proves necessary, we have said we will consider time for implementation of new arrangements," he wrote.

(Editing by Gareth Jones and Susan Fenton)