An official inquiry has been launched into how much health managers spent on bribing family doctors to meet Whitehall targets.

GPs were given £100 for each smoker they said had quit for four weeks under the scheme exposed last week by The Independent on Sunday.

Brent Teaching Primary Care Trust (PCT), which explicitly linked the incentive to improving its "star rating", is now under investigation by the North West London Strategic Health Authority.

A spokeswoman said an initial report had been sent to the Department of Health, and that it was waiting for information regarding expenditure.

There has been further evidence of inducements offered to GPs to identify smokers who had quit in order to meet trusts' quotas for reducing smoker numbers.

In Harrow, GPs have been offered £5 for each smoker they report to smoking cessation programmes run in local chemists, while in South Somerset, the local primary care trust offered £100 worth of shopping vouchers to staff who quit.

Ministers strongly disapprove of the cash incentive schemesand smoking cessation experts question how effective they are in helping people give up.

Dr Tim Lancaster, of the Cochrane Tobacco Addiction Group, said: "I'm not aware of evidence that offering cash incentives to GPs will work."

Doctors also expressed concern that data protection laws may be being be abused to meet Government targets.

Ascheme run by Brent PCT, GPs received £5 for each quitter they reported.

Dr Gillian Braunold, chair of the Brent Local Medical Committee, said: "You only need one patient to ask what else have you released about our consultation. The whole thing is farcical."