British MPs on Wednesday voted in favour of allowing fracking under national parks, despite earlier promises of a ban.

MPs voted by 298 to 261 in favour of extending legislation to permit fracking 1,200 metres (4,000 feet) below national parks, areas of outstanding natural beauty and world heritage sites with drilling taking place from outside the protected areas.

Environmental campaigners and opposition lawmakers accused the government of using "sneak" tactics to relax the legislation.

The issue was discussed and approved in committee only on Tuesday and the vote was held under a procedure normally used to speed minor parliamentary business.

Opposition MPs criticised the rush to back fracking, a way of extracting gas by pumping water, chemicals and sand underground that opponents say could pollute water supplies, scar the countryside, and trigger earthquakes.

Labour's chief energy spokeswoman Lisa Nandy accused ministers of using a "parliamentary back door" to try to approve the "weak regulations" without debate.

Liberal Democrat leader Tim Farron said the government was trying to "sneak" through a decision, labelling such a move "outrageous".

The change would allow shale gas companies to drill sideways under national parks. Before the general election, the government had made a commitment for an "outright ban" on drilling in national parks.

"It's not even a year since the government promised to ban fracking in national parks," Hannah Martin, energy campaigner at Greenpeace, said in a statement.

"Now it's trying to pull a fast one through an arcane parliamentary process by hawking out the land beneath our most beautiful landscapes to let fracking companies drill sideways deep beneath them," she said.

Friends of the Earth said the government was "pushing through" the changes without a full debate, adding that the rules could endanger drinking water aquifers.

"People won't be fooled by the government's blatant attempt to get around their promises on a technicality -- fracking under a national park does not equate to an 'outright ban'," said Rose Dickinson, energy campaigner for Friends of the Earth.

Prime Minister David Cameron's government has pledged to go "all-out for shale", saying it would increase energy security, keep prices down and create jobs.

But there is widespread opposition and there is no commercial fracking under way in Britain yet.

The government's plans were dealt a blow earlier this year when local authorities rejected plans for an exploratory fracking site by energy firm Cuadrilla in northwest England following protests.