London, May 19 (IANS): The British economy needs a net inward migration flow of 200,000 people a year if it is to avoid the "catastrophic economic consequences" linked to Brexit, a study revealed on Friday.

The Global Future study said the UK's low productivity, ageing population and shortage of labour in key areas, such as the National Health Service (NHS), show that net migration of 200,000 will be needed annually, the Guardian reported.

The report criticises Labour and the Conservatives for refusing to be honest with the British public about the level of migration the UK requires. 

It warns that if the UK refuses to be flexible about its sources of labour, it could face a decade of slow growth.

The Conservatives recommitted themselves to a target of limiting net migration to tens of thousands in their election manifesto on Thursday, promising to double the cost to an employer of hiring a skilled worker from overseas.

The report said that even with a later retirement age, Britain faces a demographic time bomb, and needs migration of 130,000 a year to maintain the working population at its current level, reports the Guardian.

"The dependency ratio -- the number of people of working age (16-64) versus those over 65 -- is worsening. Between 1950 and 2015, this fell from 5.5 to 3.5. Only the recent increase in net migration has prevented it from falling even more precipitously," it said.

"Between 2000-2050, the number of people over 65 will double, whilst the number of over-85s will quadruple. The working population would need to double in order to maintain the ratio at its current level."