THERESA May's desire for secrecy around her plans for Brexit is hindering the UK Government's ability to prepare for leaving the European Union, a think-tank has warned.

The Institute for Government[IoG] said Whitehall departments were uncertain about what to do before the Prime Minister triggered Article 50 of the EU treaties to being the Brexit process, which she has promised to do by the end of March.

The think-tank also said there was a lack of preparation for what happened on the first day the UK was officially out of the EU, expected in spring 2019, to avoid a "cliff edge" at the point of exit.

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It said the so-called Great Repeal Bill to transpose EU law into UK law would not be a simple "lift and shift" but instead would be "much more complex than anticipated" and involve "difficult" policy decisions.

Work needed to be carried out to understand what different systems and institutions would be required such as regulatory bodies or customs regimes, the report explained.

While some departments were doing this forward planning, others were more reactive and focused on responding to requests for information from the Department for Exiting the European Union.

Meantime, the IoG said Brexit threatened to create a "severe budget squeeze" on top of already planned departmental spending cuts.

The think-tank called on ministers to provide more information on the process, timelines and expectations departments needed to meet before Article 50 was triggered to ensure departments were doing sufficient planning and ensured they had sufficient staff and money.

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Hannah White for the IoG said: "We are rapidly approaching the triggering of Article 50 but our research shows that the civil service still doesn't have what it needs in terms of money, staff and information to enable politicians to get the best deal for the country.”

She added: "This is not about revealing whether we are heading towards a hard, soft or grey Brexit. This is about being ready for the negotiations, and getting ready for life after Brexit. We know the civil service has the skill to do this, now it needs clear direction from Number 10."