The mother of a boy with autism has launched a her own billboard campaign pleading with the Prime Minister to call her and discuss treatment for the condition.

Former actress and television presenter Polly Tommey, whose 12-year-old son Billy is autistic, has posted the personal message on 85 sites within the M25. In maroon italicised writing it reads "Dear Gordon, I can save you £508m a year. Please call me on 020 7147 9980... when it's convenient. Many thanks, Polly Tommey."

The number on the advertisement goes through to a voicemail message thanking the caller and then gives the website of The Autism Trust. The Prime Minister has now written to Mrs Tommey to address her concerns.

Mrs Tommey, who has worked as a body double for Charlotte Rampling, believes the Government could save £508m a year by helping more autistic adults find jobs so they no longer have to claim benefits.

The 42-year-old said that she hoped the campaign would raise awareness of the difficulties faced by those with the condition. "Autism has become a taboo subject here in the UK," she said. "It affects over half a million people and yet our Government and political leaders appear to disregard the often overwhelming stress and demands faced by families of those with autism. Unless the problems of autism are understood, unless services become more available, and unless support is more widespread, we will see an underclass of disaffected and desperate autism households emerge."

Mrs Tommey founded The Autism Trust after discovering that Billy has the condition when he was two years old. The cost of the advertising campaign is £500,000 but, contrary to news reports yesterday evening, the charity did not have to raise funds especially. It was able to secure them through donations from advertising outlets.