Researchers who trawl the internet, music festivals and "head shops" for drugs found 17 substances that had never been seen before in just over a year.
The authorities have since taken action to limit the sale of two new party drugs - known to users as Mexxy and Ivory Wave.
But the study adds that many products sold as legal highs may actually contain banned substances and that there is no guarantee they are safe to take.
It warns: "The analysis shows that just because a substance is termed 'legal' does not make it safe or 'legal' and the contents of a package are probably 'not what it says on the tin'.
"Government is concerned about the harms posed by these drugs, and the continuing rise in reports linking the use of [legal highs] to A&E presentations and some deaths."
The former Archbishop of Canterbury, Lord Carey, this week called a company that sells legal highs an "evil trade" that should be shut down.
And the Justice Secretary, Kenneth Clarke, became the first serving Cabinet minister to admit that Britain is "plainly" losing the decades-long war on drugs.
The Home Office report shows that as well as trying to stop dealers selling narcotics that have long been illegal, the authorities are also having to fight a growing trade in new substances that mimic existing recreational drugs but which are legal when they are created in labs.
"The increased development and availability of [legal highs] is changing the face of the drug scene and its 'marketplace' with greater access via the internet for both their purchase and the sharing of information in forums and blogs," the report said.
A body called Forensic Early Warning System was set up by the Home Office last year to identify legal highs as soon as they go on sale and pass on details to the Advisory Council on the Misuse of Drugs, which can tell ministers to ban a substance.
Between January 2011 and March this year, researchers obtained 1,300 samples that were analysed by scientists.
FEWS identified 17 New Psychoactive Substances (NPS) "not previously seen in the UK", more than one a month.
Eight were found online, one at a festival and eight were identified after being seized by police.
Five were synthetic forms of cannabis, two were similar to ecstasy, one had effects similar to khat leaf and four were psychedelic drugs like magic mushrooms.
The Government has recently taken action to stop two legal highs being taken, with methoxetamine (Mexxy) being made subject to a one-year ban and importation of 2-DPMP (Ivory Wave, Purple Wave, Vanilla Sky) prohibited.
However 19 per cent of all samples seized contained drugs already controlled under the law, including cocaine, ketamine and MDMA, so anyone caught in possession of them could have faced prosecution.
Some of the "very potent" legal highs contained as many as eight different substances, so "no one can really be sure what each individual package contains".
Experts want the Government to introduce blanket bans on drugs that have particular effects on the brain, so that they become illegal as soon as they are manufactured.
:: The drugs found in Britain by researchers for the first time between January 2011 and March 2012
2. MDAI (Sparkle)