The new boss of BP has promised to fundamentally "reinvent" the oil giant as he pledged the company would become carbon neutral by the middle of the century.

Bernard Looney, who took the top job last week, said that his business would aim to get to net-zero by 2050 – the same target that was adopted by the UK Government last year.

"The world's carbon budget is finite and running out fast; we need a rapid transition to net-zero. We all want energy that is reliable and affordable, but that is no longer enough. It must also be cleaner," Looney said.

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However, critics were quick to point out that the company's announcement left many unanswered questions and contained little detail on how to reach BP's aims.

"There is nothing ambitious about a plan that is simply not credible. BP's net-zero pledge looks like an attempt to grab some positive headlines by a new CEO, but with little of substance to show how it will achieve these grand claims," said Murray Worthy at Global Witness, an environmental charity.

At a speech in front of investors, analysts and the media - all of whose travel had been carbon offset - Mr Looney promised more detail in September.

"If anyone sees BP acting in a way that is counter to what I say here today then I want to hear about it," he told onlookers.

Looney will hope to leave his mark on the century-old business after taking over from American Bob Dudley last week.

In his first major announcement as chief executive, he said that BP - one of the world's oil supermajors - will eliminate the 55 million tonnes of carbon dioxide equivalent that it emits into the atmosphere each year.

READ MORE: UK oil and gas sector unveils net-zero emissions plan in Edinburgh

The net-zero policy will also cover carbon in the oil and gas that it produces, around 360 million tonnes of carbon equivalent. It will slash emissions by more than 400 million tonnes per year, or approximately the same as the UK's total annual emissions.

"This is what we mean by making BP net-zero. It directly addresses all the carbon we get out of the ground as well as all the greenhouse gases we emit from our operations," Looney said.

Mr Looney promised that BP would halve the carbon intensity of the products it sells by 2050 or sooner, while increasing investment in low-carbon alternatives.

"We expect to invest more in low-carbon businesses – and less in oil and gas – over time," he said on Wednesday.

Investors reacted well to Looney's statements, sending shares up by just under 1%, while investor groups that had been pushing for action on climate change seemed optimistic.

Stephanie Pfeifer, chief executive of the Institutional Investors Group on Climate Change (IIGCC), said: "This is a very welcome announcement from BP's new CEO. We need to see a wholesale shift to a net-zero economy by 2050. This must include oil and gas companies if we are to have any chance of successfully tackling the climate crisis."

Greenpeace said that Looney was burdening his successors with much of the responsibility to get to net-zero by setting the aim at the middle of the century, when he will have left the business.

Charlie Kronick, the organisation's oil adviser, said there were many urgent and unanswered questions: "When will they stop wasting billions on drilling for new oil and gas we can't burn?

"What is the scale and schedule for the renewables investment they barely mention? And what are they going to do this decade, when the battle to protect our climate will be won or lost?"