Rules designed to halt fracking operations if they trigger minor earthquakes could be relaxed as the shale industry begins to expand, the UK energy minister, Claire Perry, has said.

A series of small tremors seven years ago prompted tough regulations that mean even very low levels of seismic activity now require companies to suspend fracking.

The shale gas firm Cuadrilla plans to start fracking near Blackpool this week if it can see off a last-minute legal challenge on Thursday.

If seismic sensors detect anything above 0.5 magnitude on the Richter scale – far below what people can feel at the surface – the company would have to stop and review its operations.

But Perry has told a fellow Conservative MP that the monitoring system was “set at an explicitly cautious level … as we gain experience in applying these measures, the trigger levels can be adjusted upwards without compromising the effectiveness of the controls”.

The comments were made in a letter to Kevin Hollinrake, the MP for Thirsk and Malton, whose constituency has several prospective fracking sites. The letter was obtained by Greenpeace’s investigative unit, Unearthed.

Hollinrake, who is pro-fracking if it can be done safely, told the Guardian: “We’d need to be very careful about any revision to the regulations put in place. I’d want to understand why we were doing that and take plenty of evidence. We certainly wouldn’t want to see those rules being relaxed now.”

He said the limits showed the government was taking a very cautious and responsible approach to fracking.

The industry is regulated by a traffic light system introduced after the 2011 tremors. Green is zero-magnitude on the Richter scale, amber is anything up to 0.5, meaning fracking should proceed with caution and potentially at a slower rate, and red is anything at 0.5-magnitude or greater, meaning operations are suspended immediately.

Cuadrilla said it would be fracking slowly at its Preston New Road site near Blackpool because of concerns over seismic activity, meaning the process will take about two and a half months rather than the fortnight it would take in the US. The company is publishing seismicity data online.

Any relaxation of the stringent seismicity rules imposed on operators would help speed up exploratory fracking.

Experts said there was room for raising the limits because they had been set so low to start with. “That threshold [of 0.5 magnitude] is way below the size of an earthquake anyone is likely to feel. People start to feel quakes around 1.5 to 2.0, so you’ve got one order of magnitude to play around with before they are felt by people,” said Dr Brian Baptie, a seismologist at the British Geological Survey.

If the limit was raised to 1.5, it would be on a par with two of the tremors felt by people near Blackpool in 2011. But it would still be far from the level where a quake would do any damage to buildings, and below the thresholds for blasts during quarrying, which can go up to 2.0.

Baptie said: “There is some leeway to adjust them [the trigger points] later. The trade-off is for earthquakes not to be a problem and for the industry to be able to work efficiently.”

However, Kate Blagojevic, Greenpeace’s head of energy and climate, said: “Given that earthquakes were one of the reasons that led to a seven-year hiatus in fracking, it’s understandable that the government want to boost their pet project by diluting the standards on protection from seismic activity.”

She pointed to problems with fracking in the light of this week’s UN Intergovernmental Panel on Climate Change report, which said there were only 12 years left for global warming to be kept to a maximum of 1.5C if the risks of drought, floods, extreme heat and poverty for hundreds of millions of people were not to worsen.

“Given yesterday’s climate report, it’s crucial that the fracking industry is stopped permanently and the government invests in energy efficiency and renewables so that we don’t need to use so much gas to supply our homes, offices and buildings,” she said.

Rebecca Long-Bailey, Labour’s shadow business secretary, said: “The government’s answer to earthquakes caused by fracking seems to be to relax the rules on earthquakes. This is a government determined to push fracking at any cost.”

The Labour MP visited Cuadrilla’s Preston New Road site in Lancashire on Monday, meeting with local people concerned at the impact of fracking.

Cuadrilla’s hopes of beginning fracking this week hinge on whether it can fend off an eleventh-hour legal challenge brought by a retired Lancashire businessman, Bob Bennett, against Lancashire county council.

On Thursday at 2pm, the high court will hear evidence from lawyers for the council, the company and Bennett, over whether sufficient measures are in place in the case of an emergency at the site.

“People’s concerns about how to evacuate the area in the event of a well blowout or gas leak at the fracking are being sidelined by Lancashire county council”, said Bennett.

The government said the rules around seismicity could not be changed without a consultation.

A spokesperson at the Department for Business, Energy and Industrial Strategy said: “The UK’s world class oil and gas regulations, which have a track record of success that goes back decades, will ensure that shale development can only happen safely and in an environmentally responsible way.

“The seismicity levels were set low as a precautionary measure and may be reviewed in the future subject to scientific advice.”