She maintained that her view of future British-EU ties was credible and expressed confidence that she would reach a good deal, although she appealed for the bloc to be more flexible in its negotiations.
Lord Heseltine is the latest senior figure to weigh in to the Brexit debate after former prime ministers Sir John Major and Tony Blair delivered speeches earlier this week warning of the damage leaving would cause.
During her speech, May also reiterated that leaving the European Union also means leaving the digital single market. "Life is going to be different".
British Prime Minister Theresa May confirmed that her country is leaving the single market, and the mutual commercial access of Brussels and London will be "smaller than now".
Setting out her plans for the future partnership between the United Kingdom and the EU, May highlighted the need for ensuring a data protection deal is in place between the United Kingdom and the EU.
That includes the European Medicines Agency, which is relocating its headquarters from London to Amsterdam because of Brexit.
"She gave a lot more detail in this speech than she has in any other speech and she indicated her intention to concede some ground in relation to the European Court of Justice, paying into the agencies, regulatory standards and developing some kind of customs arrangement".
If progress is to be made, Mrs May will have to firmly distance herself from Mr Johnson and demonstrate instead that she fully understands the dangers which were so starkly set out by Mr Mitchell.
Despite her efforts to quell fears of a hard border in Ireland, she accidentally began a sentence: "To achieve a hard border-".
Mrs May backed "new technology" to help keep the border between Northern Ireland and the Republic as "soft" as possible. This is also the main message she sends to the rival factions in her Conservative Party - London will not get everything it wants and she is ready to put the interests of business and jobs before ideology.
A report issued by the House of Lords in January argued that Ireland's SEM would need to be maintained, but also warned of the need for "careful consideration" owing to how some EU-led energy laws would need to be adopted in Northern Ireland for that to occur.
There will be concern in Brussels that her cherry picking threatens to unpick the legal and regulatory integrity of the single market - and it is something that officials have said all along that they can not countenance in the same way she has always been clear that leaving the European Union means quitting the single market and taking back control of Britain's "money, borders and laws".
"It is because of the absence of a clear understanding of what the British Government is asking for in terms of a future relationship that we have had to insist on a default position in the context of the unique challenges that Ireland faces", he said.
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