Mexico City - In a surprise reversal, Mexican President Vicente Fox will not sign widely criticised narcotics legislation to decriminalise the possession of small quantities of marijuana, cocaine and heroin, his office said on Wednesday.

The president's office said the law, which also toughened sentences for dealing and holding larger amounts of the intoxicants, would be sent back to Congress for revision.

"In our country the possession of drugs and their consumption are, and will continue to be, crimes," the office said in a statement.

Fox's decision was unexpected, given that the legislation was initially designed by his office and introduced by his party. This week, his spokesperson praised the law and insisted the president would quickly sign it, despite rumblings of discontent from Washington.

The legislation, passed by Congress last week, shocked Mexico's northern neighbour, which counts on its support in a war against gangs that move massive quantities of cocaine, heroin, marijuana and methamphetamines through Mexico to US consumers.

It was also criticised by authorities in Mexican tourist towns who worried about a flood of hard-partying US thrill seekers attracted by the new, lenient rules.