A blanket ban on so-called legal highs comes into force on Thursday, amid questions over how far it will deter users and warnings it could drive dealers on to the dark web.

Laws criminalising the production, distribution, sale and supply of the drugs took effect from midnight.

Offenders will face up to seven years in prison under the Psychoactive Substances Act. It had been widely expected that the measures would be rolled out in April but the start date was pushed back.

Ahead of the ban, a survey by the YMCA suggested that while overall usage is likely to decrease, around two thirds of young people who currently take the drugs are likely to continue using them in the future.

Meanwhile, there have been warnings that the ban could drive dealers on to the dark web - selling on unlisted websites that are difficult to trace.

Commander Simon Bray, the National Police Chiefs Council's lead on New Psychoactive Substances, said: "Clearly, there will be some movement onto the dark net, where people find it lucrative to sell substances and where people are going to buy them."

New psychoactive substances saw an explosion in popularity in around 2008 and 2009. They contain substances which mimic the effects of drugs such as cocaine, cannabis and ecstasy.

The new Act states that a substance produces a psychoactive effect "if, by stimulating or depressing the person's central nervous system, it affects the person's mental functioning or emotional state".

A number of substances, including food, alcohol, tobacco, nicotine, caffeine, alkyl nitrites (poppers) and officially-sanctioned medical products are excluded from the legislation.

The Bill has come under intense scrutiny since it was first proposed by the Government in 2015 and had been criticised as lacking clarity and infringing civil liberties.

Barrister Matthew Scott wrote in The Telegraph that the legislation sought to "ban pleasure".

He wrote: "Did you know that tea was a 'psychoactive substance'? Well under this new law it will be, and you will be allowed to drink it only as a special exemption from the normal rule.

"Whatever the wisdom of the goal, the Government seems to have decided that banning 500 substances is not enough. It must ban almost everything that gives pleasure.

"And what a ban. Of all the many idiotic, ill thought out and pointless laws ever passed, this would be one of the silliest."

Tejinder Reehal, who manages a shop that has sold legal highs, said: "I think the ban is going to drive people underground - we have seen it before with mushrooms and mcat.

"It is good in a way and bad in a way. The most serious stuff will come off the market, but the milder stuff should be allowed to be sold, as these are just used by normal, working people for recreation.

"I am strict about not selling to under-18s, but there is no way you can regulate the internet."

Minister Karen Bradley said: "The Psychoactive Substances Act sends a clear message - these drugs are not legal, they are not safe and we will not allow them to be sold in this country."