Boeing reportedly sold the 737 Max planes that crashed in Indonesia and Ethiopia without two safety features that the US aircraft manufacturer offers airlines for an additional cost.

It is still not clear what caused Lion Air flight 610 and Ethiopian Airlines flight 302 to crash, but investigators have said flight data showed clear similarities between the two incidents.

The two safety features in question were an “angle of attack indicator” and an “angle of attack disagree light”, both of which were not included in the aircraft by Boeing as standard safety features, according to a report in the New York Times.

Reports on Thursday night suggested that Boeing will make at lelast one of the safety features – an indicator light – standard on the 737 Max 8 aircraft, according to unnamed officials cited by Associated Press.

It is up to airlines to decide whether to pay for upgrades to a standard plane – a practice that is common among aircraft manufacturers and allows them to charge extra, often for aesthetic features relating to seating or lighting, but for other features relevant to the operation of planes, too.

Regulators do not require airlines to buy optional extras, and many low-cost carriers opt not to.

Investigators are looking into whether faulty data from sensors on the Lion Air plane may have caused a new software system, known as the manoeuvring characteristics augmentation system (MCAS), to malfunction.

MCAS takes readings from two angle of attack sensors to determine how much the plane’s nose is pointing up or down. If the software detects the nose is pointing up at a dangerous angle it has the ability to automatically push the nose down in an attempt to stop the plane stalling.

One of the optional extras – the angle of attack indicator – displays the readings of the two sensors, and the other – the disagree light – is an alert that activates if those sensors do not agree.

Boeing has told airlines that it expects to have new software ready by the end of the month, and it is understood that the disagree light will become a standard feature on all new 737 Max planes.

The angle of attack indicator will remain an optional extra that airlines can buy, according to the New York Times report.

“They’re critical, and cost almost nothing for the airlines to install,” Bjorn Fehrm, an analyst at the aviation consultancy Leeham, told the newspaper. “Boeing charges for them because it can. But they’re vital for safety.”

The Guardian has approached Boeing for comment.

Boeing employees work on a 737 Max aircraft at the company’s factory in Renton, Washington. Photograph: Stephen Brashear/Getty Images

Voice recordings from Lion Air flight 610, yet to be officially released, suggest the pilots looked through the flight manual as the jet incorrectly alerted them it was stalling and automatically pushed the nose down. As the captain fought to climb, the software system continued to push the nose down.

Lion Air flight 610 crashed in Indonesia in October, killing all 189 people on board. Five months later, on 10 March, Ethiopian Airlines flight 302 came down, killing 157 people with no survivors.

The second fatal crash prompted fears that there was a safety fault with the model, and since then more than 300 737 Max planes have been taken out of service, and deliveries have been suspended on a further 5,000 on order.

In the UK, the Civil Aviation Authority has ruled the Boeing model is not permitted to take off, land or fly within UK airspace.

The transport secretary, Chris Grayling, said on Thursday the 737 Max would be grounded until “the problem is solved”. Speaking in the House of Commons, he said: “This country was one of the first to ground the 737 Max planes. That is absolutely the right thing to do. There are clearly some very alarming circumstances around the two accidents that have taken place.

“It is something that Boeing clearly have to deal with because, unless and until the problem is solved, I can’t see countries like ours allowing those planes to fly again.”