The rules, a fresh bid to put people off smoking, also end the sale of smaller packets of 10 cigarettes.
The greenish-brown colour for packaging has been described by some experts as "the ugliest colour in the world".
Similar restrictions will apply to e-cigarettes and packets of rolling tobacco too.
There will also be a ban on flavoured cigarettes and tobacco.
The UK-wide move is believed to lead to 300,000 fewer smokers in the UK over the next year, a review has suggested.
The findings were backed up by a report from the Australian government, which showed a similar drop in smoking prevalence - 0.55 per cent - following the introduction of plain packaging there in 2012.