A HISTORIC deal on Iran's nuclear programme could be disastrous for oil industry in the North Sea, union leaders have warned.

The accord over Iran's nuclear programme has heightened expectations of a wave of cheap oil flooding global markets after sanctions are lifted, with some experts warning in a plunge in prices.

North Sea Brent crude futures were down 3.6 per cent at $54.95 a barrel, after hitting a session low of $54.45 on as Iran and six leading world powers agreed to resume talks on curbing the country's nuclear programme.

Industry body Oil and Gas UK has estimated that around 20per cent of UK production is uneconomic at a $50-per-barrel oil price.

Hailed by US President Barack Obama as a "historic agreement", the Iran framework over limits to its nuclear capabilities provides the basis for a more comprehensive agreement that is hoped to be reached by June 30.

Low oil prices have previously led to warnings about jobs and tax revenues from North Sea oil and gas companies already dealing with rising costs and have led to widespread cuts.

Chancellor George Osborne recently gave the industry a £1.3 billion tax break to boost growth and sustainability.

Iran is understood to have a glut of unsold oil ready to hit markets and is thought to have some 30 million barrels held in a fleet of tankers. However, Tehran is prepared to produce up to 1 million barrels a day.

Before implementation of sanctions imposed by the EU and US in the middle of 2012, Iran was exporting 2.5 million barrels.

Jake Molloy, regional organiser of the RMT union's offshore energy branch who has been involved in the offshore industry for over 30 years said without intervention from government, the Oil and Gas Authority regulator or the Organization of the Petroleum Exporting Countries (Opec) to show restraint in production, the outlook for the North Sea industry with falling oil prices was grim.

"If the oil price continues to go down, there will come a point when the good old boys in Houston pull out the red pen and just put a line through it. It is no longer economic at any costs so they'll shut down.

"And that spells disaster for the UK as a whole because if these oil companies aren't going to support continued production of marginal fields then it gets left in the ground.

"We would say Opec should show some restraint in production, and create an environment that sustains production across the globe. But they have never shown a willingness to engage in that kind of dialogue before.

"Opec would just tell us to get lost, because the Saudis have made it quite clear that their break even position is round about the $25 to $30 a barrel mark. It is similar to Iran and the whole Middle East. They don't have added costs of flying or shipping to and from and operating in a hostile environment, to the extent we have.

"It could be disaster not just for the UK but Norway, Denmark, Holland, Russia especially where they are already struggling."

Some analysts say low oil prices boosts economic growth in many countries due to lower costs for businesses and consumers.

Bank of England governor, Mark Carney, has previously described the sharp fall in oil prices as "unambiguously positive" for the global economy and for the UK.

Naeem Aslam, chief market analyst at AvaTrade said: "The Iran nuclear deal is a massive blow for the oil price and we could see the crude-oil price falling to $30 very easily. This deal actually represents 1 million barrels a day of extra oil on the market so net effect on the supply equation will be nearly 2 million (barrels a day)."

A spokeswoman for Oil & Gas UK said: "The sector is now working hard to restore competitiveness by taking measures to improve its cost efficiency and build on the foundations of the positive tax changes announced in the Budget and the establishment of the new regulator, the Oil and Gas Authority, so that it can sustain high-skilled jobs, investment and exports for decades to come."