The Conservative government has yet to set out its vision for Brexit following the referendum vote on June 23, beyond saying it would not start formal exit negotiations with Brussels until next year.
"I know many people are keen to see rapid progress and to understand what post-Brexit Britain will look like. We are getting on with that vital work," May told the House of Commons.
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But she said: "We will not take decisions until we are ready. We will not reveal our hand prematurely and we will not provide a running commentary on every twist and turn of the negotiation".
Labour party leader Jeremy Corbyn said there was "huge uncertainty" and an "extraordinary lack of planning", warning also about rising levels of hate crimes against EU immigrants.
Corbyn warned that "parliament and the public cannot be sidelined" on such a major constitutional change.
Scottish National Party lawmaker Angus Robertson challenged May to give an "in or an out answer" about whether Britain would remain a full member of the European single market.
She declined, saying: "It would not be right for us to prejudge those negotiations.
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"We will be ensuring that we seize the opportunities for growth and prosperity across the whole UK, including growth and prosperity in Scotland."
May has highlighted positive signals from India, Mexico, South Korea, Singapore and particularly Australia as proof that Britain will be able to forge new trade deals outside the EU.
But Australian trade minister Steven Ciobo warned in a speech in London late Tuesday that negotiations on a bilateral agreement may be "a few years off" -- and said that his country would prioritise a deal with the EU.
"We're working on an Australia-EU free trade agreement, which will prepare the way for our own agreement in years to come. The best deals are those done quickly and our talks with the EU are more advanced," he told an audience at Asia House.
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