Fierce Heathrow expansion opponent Boris Johnson has invested millions of pounds of public money into the airport.

The Mayor of London gave the go-ahead for the Greater London Authority (GLA) to purchase a £3.5m corporate bond, according to official documents.

Teddington Councillor and GLA member Stephen Knight obtained the investment records in the same week the decision on Heathrow's expansion was delayed by six months.

During today's Mayor's Question Time, Coun Knight tweeted: "Boris sees nothing wrong with opposing 3rd runway whilst investing taxpayers' money in Heathrow bonds to be used to finance it."

Mr Johnson told the chamber that he "does not direct GLA funding", which Coun Knight's Lib Dem colleague Caroline Pigeon branded "hypocritical".

The bond is due to mature next March, according to the Independent on Sunday, delivering a £11,000 return to the GLA before the airport expansion deadline in June 2016.

The Mayor, who must personally approve the overall investment strategy, has been a vocal opponent to Heathrow's £23bn expansion bid.

Coun Knight said: "It beggars belief that Boris Johnson whilst publicly campaigning against a third runway at Heathrow, has behind-the-scenes poured over £3.5m of London taxpayer's money into Heathrow bonds, which will help finance the expansion.

"When I put this hypocrisy to him at Mayor's Question Time, the Mayor simply denies there is any contradiction, despite Heathrow's prospectus for its corporate bond issue explicitly referring to the need to raise capital for the third runway.

"Many Londoners will also be shocked to discover that the Mayor is also investing over £100 million of taxpayers' money in a Saudi Arabian bank.

"I believe it is time the Mayor started taking reputational risk into account when making investment decisions with our money."

Other GLA investments laid bare in the revealing documents show £200m of taxpayer's money being placed with banks in the Middle East.

The GLA's Group Investment Syndicate (GIS) has an annual budget of £17bn.