But the doomed helicopter was still allowed to fly because decisions on whether to take-off are down to individual pilots.

Helicopters over London follow strict rules and guidelines from the Civil Aviation Authority and navigation service NATS – but the final decision on flights rests with the pilot.

One qualified pilot told the Enquirer: “Only the very best and most experienced pilots would consider going up in conditions like that.

“Cloud cover was really low and even police helicopters were grounded due to the weather.

“The cloud level was 100ft in London that morning – never mind the fog and I wouldn’t be surprised if he was the only helicopter up that morning as most pilots would not have taken off.

“Battersea is very tight to get in and out of and it’s very easy to get disorientated in those type of conditions.

“If you are in clouds you rely on your instruments and that depends on them being reliable.

“But if you can see you look out the window but you can’t necessarily see everything when the weather is bad.

“Weather can sometimes be very localised so conditions may have been better where he took off in Redhill and then it deteriorated.

“The authorities might have to consider insisting that all helicopter flights in London are in VFR (visual flight rule conditions) apart from the emergency services.

“That would be sensible with all the tall buildings going up.”

The dead pilot has been named as Capt Peter Barnes.

His helicopter crashed into a tower crane at Berkeley Homes’ St George Wharf site yesterday morning at 8 o’clock.

Passer-by Matthew Wood,39, from Sutton was also killed.

The crane driver and his mate had turned up late for work and were preparing to climb up to the cab when the helicopter hit.

But Berkeley chairman Tony Pidgley told the BBC the crane driver would not have been able to start lifting anyway because of the “fog level”.

He said: “The operative is just not allowed up that crane in conditions like that because you just can’t see.”

London Mayor Boris Johnson has confirmed a review of warning lights on cranes and tall buildings in the wake of the tragedy while David Cameron will review rules over flights across central London.

Crash investigators will now be examining all possible causes.

A NATS spokesman said: “Earlier in the helicopter’s journey the pilot had been receiving an air traffic control service from NATS, although was not receiving a service at the time of the crash.”

The Civil Aviation Authority said: “There are requirements for lighting on tall structures. In addition, where appropriate, very tall structures are also notified to pilots for flight planning purposes, as was the case with the crane that was involved in this accident.

“The independent Air Accidents Investigation Branch is now investigating today’s accident to establish the facts. The CAA will be providing any assistance required.”