The former head of MI6 has urged the government to block an American takeover of British aerospace giant Cobham.

Sir John Sawers, who ran the spy agency from 2009 until 2014, told a conference in London that “we need to be able to say for firms like Cobham, actually we want to keep this technology in the UK”.

His intervention is the most high-profile challenge yet to a £4bn takeover of the 85-year-old business by American private equity firm Advent, and follows a string of warnings that losing such a strategically vital asset will damage the Britain's international standing.

The sale was overwhelmingly approved by Cobham shareholders last month but has yet to be signed off by the Government, which could choose to block it on national security grounds.

It would see Advent gain ownership of Cobham’s lucrative defence projects, including radar, secure radio systems and its world-leading air-to-air refuelling systems.

Sir John said that losing the business would give Britain less ability to influence American decisions - creating serious problems because the US is so unpredictable under President Donald Trump.