Theresa May has set Britain on course for a hard Brexit - but failed to address the issue of what happens if Parliament blocks her deal with the EU.

The Prime Minister outlined the 12 principles that will govern her EU negotiations for the next two years, including confirmation that Britain will be giving up its membership of the European single market.

In a key development, Ms May said the final deal she reaches with EU leaders will be put to a vote in both the House of Commons and the House of Lords.

But it immediately raised questions as to what happens if Parliament votes to reject the deal, two years after the UK triggers Article 50.

ITV's Robert Peston asked "would we still be in the EU" if there is a no vote in Parliament.

Ms May appeared to dodge the question, saying: "What does it mean? There would be a vote for the British Parliament when it comes to the deal, as I've said.

"If you think about it, the British Parliament voted overwhelmingly for this to be the decision of the British people," she said. "The British Parliament voted recently effectively for the Government to get on with it.

"I'm sure that the British Parliament will want to deliver on the views of the British people and respect the democratic decision that was taken."

Ms May warned the EU against trying to "punish" the UK as a warning to others who might seek to leave the bloc.

She said seeking a punitive Brexit deal with the UK would be "an act of calamitous self-harm", adding that "no deal for Britain is better than a bad deal for Britain".

Ms May said she did not want a long transitional period to move towards Britain's new relationship with the EU.

But she said that once a deal is agreed, it will be implemented with a "phased approach, delivering a smooth and orderly Brexit".

She said: "We will seek to avoid a disruptive cliff edge and we will do everything we can to phase in the new arrangements we require as Britain and the EU move towards our new partnership."

Listing her key objectives for the Brexit negotiations, Ms May said: "These are the objectives we have set:

"Certainty wherever possible. Control of our own laws. Strengthening the United Kingdom.

"Maintaining the common travel area with Ireland. Control of immigration. Rights for EU nationals in Britain and British nationals in the EU. Enhancing rights for workers.

"Free trade with European markets. New trade agreements with other countries. A leading role in science and innovation. Co-operation on crime, terrorism and foreign affairs. And a phased approach, delivering a smooth and orderly Brexit."

And she added: "This is the framework of a deal which will herald a new partnership between the UK and the EU. It is a comprehensive and carefully considered plan that focuses on the ends, not just the means, with its eyes fixed firmly on the future and on the kind of country we will be once we leave."

The Foreign Secretary, Boris Johnson, attended Ms May's speech at Lancaster House. He said the "powerful" address would be "well received" by EU nations.

"Because it's negotiable, this is something that I think will be good for the UK and good for the rest of the EU as well," he said.

Asked why the EU would give the UK a "free lunch", Mr Johnson said: "As the Prime Minister said, I think it's going to be good for both sides."

He went on: "We very strongly think this is in our mutual interest. We're not leaving Europe, we're disentangling ourselves from the treaties of the EU. We can remain powerfully committed to Europe with a new European partnership ... whilst also going forward with an identity as Global Britain."