The unit was used to encourage compliance with coronavirus regulations during the pandemic.

One advert showed a close-up photo of an intensive care patient in a mask, with the caption: “Look her in the eyes and tell her you never bend the rules.”

Another said: “Look him in the eyes and tell him you always keep a safe distance”.

The letter drew attention to a government memo from March 2020, which suggested that “the perceived level of personal threat needs to be increased among those who are complacent” and called for more frightening messaging.

The Telegraph understands that Parliament’s Public Administration and Constitutional Affairs Committee will investigate the use of the behavioural insights team as part of its investigation into the Government’s activities during the pandemic. It will coincide with the second anniversary of the first lockdown.

William Wragg, the committee’s chairman, said: “I think the central issue is how ‘nudge’ sits within parliamentary democracy and ministerial accountability.

“Normally, it’s quite straightforward to know where lines of accountability are between the law, parliamentarians scrutinising the law and the public following it.

“And this is a wider question of how much, in a parliamentary democracy, sits outside of that approach.”

The psychologists also warned that “scare ads” have had unintended consequences.

“Shaming and scapegoating have emboldened some people to harass those unable or unwilling to wear a face covering,” they wrote.

“More disturbingly, the inflated fear levels will have significantly contributed to the many thousands of excess non-Covid deaths that have occurred in people’s homes, the strategically-increased anxieties discouraging many from seeking help for other illnesses.”