The best international scientists are being deterred from heading to the UK due to a Foreign, Commonwealth and Development Office vetting scheme.

The Atas scheme is designed to prevent the export of technology with potential military applications and was greatly expanded last year. Some people are unable to take up prestigious posts for up to seven months because of the lengthy delays.

The foreign secretary of the Royal Society said that it was acting as a deterrent. Great talent doesn't stop working. The US, Canada, France, Germany, and other countries are going to be looking to attract the same talent as we are and this is a huge national risk.

EU and US academics are exempt from the scheme because they are not considered to have military applications. Most engineering, computing and materials science are included in this.

Academics on skilled worker visas, including those already in the UK, were added to the list of people who needed to apply last year.

More than half of the academics who left the UK in the past year were from China.

The FCDO told the Guardian that the majority of applications were processed within 30 days.

Some are facing delays. Those who are being delayed are being accommodated by universities. The chief executive of Universities UK said that some timescales are very significant and leave people in limbo for a long time.

Concerns about the process are also present. No reason is given for a rejection and there is no appeals process.

A PhD student had to wait five months for clearance after being recruited by Prof Alasdair McDonald. It is as if there is a machine learning technique in charge of Atas that waits for some time between four and seven months and randomly rejects a certain percentage of applicants without a clear reason.

It's hard to find out anything about the decision making process. The star chamber is similar. He said there was a question mark over whether it was under-resourced. Is it trying to reduce the number of people?

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Scientists were frustrated by the process. One Indian physicist, who wanted to remain anonymous, had his Atas application approved recently after waiting seven months, during which time he compiled a spreadsheet of 200 fellow applicants, some of whom were facing similar delays. He said that he has been in contact with many young scientists. Since they've been waiting so long, they hold resentment.

Another Indian scientist who applied at the beginning of August is still waiting to take up a post in Cambridge after completing her PhD at a top French institute, winning a prestigious international prize and turning down a position in Switzerland.

An Egyptian engineer who has been in the country for five years has been unable to leave because his visa expired while he was waiting for clearance.

The cost of UK visas and rhetoric from the home secretary on immigration gave a perception that international scientists were not welcome in the UK. He said that they were having to assure candidates that they really wanted them. The government's wish to reduce immigration doesn't relate to science talent coming to the UK

The majority of Atas applications are processed within 30 days according to a government spokesman. We would encourage students to apply early because waiting times can be longer.