Adverts proclaiming the benefits of part-privatisation of the London Underground misled the public, a watchdog has ruled.

The Advertising Standards Authority upheld a complaint by London's Transport Commissioner Bob Kiley who claimed two newspaper adverts were "misleading by inaccuracy, ambiguity, exaggeration and omission".

The ASA found the adverts were "misleading" on six counts and warned LU to "take more care in future when preparing similar advertisements".

The controversial �16bn Public Private Partnership (PPP) to revamp the network will give two private sector consortia 30-year infrastructure leases, with trains run by LU and supervised by Transport for London.

It is vital information on this controversial PPP is presented to the public in the most transparent manner

Bob Kiley

Promises in LU's adverts, published in February and March, hinged on completion of the PPP contracts but financial closure is not expected until "late summer", according to LU.

It has been delayed by London Mayor Ken Livingstone's judicial review in the High Court, which will be heard next week, aimed at blocking the scheme.

LU's advert claimed "we'll spend �16bn to fix the Tube over the next 15 years".

It suggested LU already had funding when neither the private sector bidders' offers nor the financial content of the contracts had been finalised, the ASA said.

LU also claimed that the renewal of 7.5 kilometres (4.6 miles) of the Bakerloo line was guaranteed but this was not possible as the PPP contract negotiations were continuing, the ASA said.

LU also made unsubstantiated claims it could guarantee at least �1bn would spent annually on maintenance and development.

Transparency is vital, says Bob Kiley

LU could not prove that another 17 stations would be accessible to people with mobility difficulties inside three years because the PPP contracts had not been concluded, the ASA said.

The company should also consult the Committee of Advertising Practice Copy Advice team before advertising again, the ASA said.

Bob Kiley, who believes the adverts had been used to try to garner public support for PPP, said LU had chosen to "obscure the facts with over-optimistic spin".

He said: "This PPP is massively complex and will cost taxpayers billions of pounds.

"The public has a right to know where that money is being spent.

"It is vital information on this controversial PPP is presented to the public in the most transparent manner and that it stands up to independent scrutiny."

Defiant

Ken Livingstone said: "London Underground has misled the public because this PPP cannot withstand the challenges of objective examination.

"It is unsafe, exorbitantly expensive and will not deliver the Tube improvements London was promised."

LU remained defiant over the ruling.

A spokesperson said: "We stand by everything we said and look forward to seeing the start of the Tube's transformation, as detailed in the ads, begin to take shape later this year."