Theresa May has proposed remaining in the single market for a transition period of around two years after Britain leaves the European Union.

In a landmark Brexit speech in Florence, the Prime Minister said existing market arrangements should continue to apply for an implementation period after Britain leaves the bloc in March 2019.

She indicated freedom of movement would also continue during the transition period but workers coming into the UK from the EU would have to go through a "registration system".

She also laid out plans for a new security treaty with the EU, which would bolster co-operation on security, law enforcement and criminal justice, illustrating the UK was "unconditionally committed" to the security of Europe.

Speaking in a church in the Italian city, Ms May said: "Clearly people, businesses and public services should only have to plan for one set of changes in the relationship between the UK and the EU.

"So during the implementation period access to one another’s markets should continue on current terms and Britain also should continue to take part in existing security measures. And I know businesses, in particular, would welcome the certainty this would provide.

"The framework for this strictly time-limited period, which can be agreed under Article 50, would be the existing structure of EU rules and regulations."

She moved to reassure Brexiteers that there would be a "clear double lock" on the transition period to ensure it "will not go on for ever".

Keeping the existing relationship with the EU would mean the UK has to accept continuing freedom of movement rules until up to 2021, which give ministers time to bring in a new immigration system, she said.

She offered to guarantee the rights of EU nationals living in Britain and said UK courts would consider the judgements of the European Court of Justice (ECJ) when making rulings on citizens rights.

Ms May also suggested a new mechanism to resolve disputes over the implementation of the withdrawal agreement rather than allowing the ECJ to be the final arbiter.

During her 35-minute address, the Prime Minister ruled out pursuing a Canada or Norway-style trade deal rather than a bespoke arrangement, saying: "We can do so much better than that."

She said: "One way of approaching this question is to put forward a stark and unimaginative choice between two models: either something based on EEA membership or a traditional Free Trade Agreement, such as that the EU has recently negotiated with Canada.

"I don’t believe either of these options would be best for the UK or best for the European Union."

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In an effort to kickstart flagging negotiations with Brussels, she said Britain would honour its financial commitments to the EU but failed to put a figure on the final offer, which is rumoured to be around £20bn.

Ms May said: "I do not want our partners to fear that they will need to pay more or receive less over the remainder of the current budget plan as a result of our decision to leave. The UK will honour commitments we have made during the period of our membership.

"And as we move forwards, we will also want to continue working together in ways that promote the long-term economic development of our continent."