Theresa May. PA LONDON — Prime Minister Theresa May is expected to be more direct in her threats to pull Britain out of the European Union's single market if not granted full control over immigration following Brexit talks.

That is according to a report in The Telegraph which said she will outline her "vision for Britain outside of the EU" in a key speech to be delivered later in January this year.

The contents of the speech are tipped to focus on immigration and the economy.

The report also says that the Britain's Brexit secretary David Davis and Foreign Secretary Boris Johnson will have "significant" input into what May says, while Trade Secretary Liam Fox is apparently being sidelined.

Britain voted for a Brexit by a slim majority on June 23 and, since then, there has been much speculation on when May will trigger Article 50 and therefore start the two-year negotiation period. March 2017 is the current target date but a Supreme Court case will rule in January 2017 whether she will have to get permission from parliament to do this. This could slow things down.

May says she will not give a "running commentary" on how negotiations are going but she has made it clear in various speeches that her government is prioritising immigration restrictions. This would imply a "hard Brexit" because the EU's official line is that it will not allow the UK to curb immigration and keep membership of the single market at the same time.

A "hard Brexit" is the UK leaving the European Union without unfettered access to the single market, in exchange for full control over immigration. It is seen as a potential death blow to the City of London, and therefore the wider economy, as it would mean companies would lose financial passporting rights.

If the passport is taken away, then London could cease to be the most important financial centre in Europe, costing the UK thousands of jobs and billions in revenues. Around 5,500 firms registered in the UK rely on the EU's passporting rights for the financial services sector, and they turn over about £9 billion in revenue.

May's speech later this month could be the first time she outlines concrete plans for how Britain will approach Brexit talks.

When Sir Ivan Rogers quit as the British ambassador to the European Union on Tuesday, he called on former colleagues to challenge "muddled thinking" and "ill-founded arguments" during the Brexit talks.

His 1,400-word resignation letter contained thinly veiled attacks on the way the government was so far handling the Brexit process, such as " serious multilateral negotiating experience is in short supply in Whitehall."

It also made clear that the government has not communicated to its top negotiators what the UK's strategy will be in the Article 50 talks even though they are due to start in March.

Rogers' replacement, former UK ambassador to Russia, Sir Tim Barrow, will be the government's man in Brussels when talks begin.