Image copyright Getty Images Image caption Activists are putting pressure on companies with ties to the NRA, a powerful gun lobby

A growing number of companies have cut ties with the National Rifle Association amid calls for a boycott of businesses linked to the US gun lobby after the Florida school shooting.

United and Delta airlines joined car rental giants Hertz and Enterprise in ending discounts for NRA members.

The murder of 17 people has prompted renewed calls for tighter gun controls.

Florida Republican Governor Rick Scott has backed calls to raise the minimum age for buying a gun from 18 to 21.

Mr Scott has been widely seen as an ally of the NRA who has previously opposed stricter laws in the state. However, he has come under mounting pressure to respond to the demands of students who survived the shootings at Marjory Stoneman Douglas High School.

How did the boycott come about?

Activists have tried to put pressure on the NRA since the shooting by targeting firms that offer discounts and other benefits to its members.

They have flooded its corporate partners with comments on social media under the hashtag #BoycottNRA.

Firms under pressure include delivery company FedEx and tech giants such as Amazon, which distributes NRA television programmes.

Which companies have cut NRA ties?

On Thursday, the family-owned First National Bank of Omaha said it would not renew NRA-branded credit cards, citing "customer feedback".

Enterprise Holdings, which owns the rental car brands Alamo, Enterprise and National, also said discounts offered to NRA members would end on 26 March.

The firm, which announced the move in response to comments on Twitter, declined to say why it had taken such a step but told a customer that the firm doesn't "sponsor, endorse or take a political stance on any organizations."

Other companies distanced themselves from the NRA on Friday and Saturday.

Those included MetLife Insurance, the Avis Budget Group, home security firm Simplisafe, two moving brands - Allied Van Lines and northAmerican Van Lines - and Symantec Corp which had offered discounts for its LifeLock identity theft product.

Media playback is unsupported on your device Media caption How young conservatives view #NeverAgain

Insurance firm Chubb also said it had stopped underwriting an NRA-branded insurance policy three months ago.

Delta Air Lines and United followed suit on Saturday, saying that they would ask the NRA to remove their information from its website. Both airlines had been offering special flight discounts to NRA members travelling to the association's annual meeting in May.

In Florida, the president of the Florida Education Association, which represents teachers' unions, also called on the state to look at pension holdings in gun companies in a statement to the Miami Herald newspaper.

What has the NRA said?

The NRA, which claims five million members, shot back at those corporations on Saturday, saying the moves punished the organisation's members in a "shameful display of political and civic cowardice".

"In time, these brands will be replaced by others who recognize that patriotism and determined commitment to Constitutional freedoms are characteristics of a marketplace they very much want to serve," the NRA said.

"Let it be absolutely clear. The loss of a discount will neither scare nor distract one single NRA member from our mission to stand and defend the individual freedoms that have always made America the greatest nation in the world."

The NRA has previously said people upset about the shooting should focus on lapses by law enforcement and mental health professionals.

Media playback is unsupported on your device Media caption Seven things the NRA blames after Florida

NRA chief executive Wayne LaPierre last week said "opportunists" were using the 14 February tragedy to expand gun control and abolish US gun rights.

"They hate the NRA. They hate the second amendment. They hate individual freedom," he said.

Prior campaigns aimed at the NRA have had limited results.

President Donald Trump has defended the NRA, while others criticised the boycott on Twitter.

What about gun purchases?

On Friday, the governor of Florida urged state lawmakers to restrict access to firearms for young people and the mentally ill.

Rick Scott said his aim was for Florida to "require all individuals purchasing firearms to be 21 or older," adding that the he wanted to make it "virtually impossible" for anyone with mental health issues to acquire a gun.

Mr Trump earlier said he supported the proposal to raise the age at which a person can buy a gun from 18, and said he believed the NRA would back such a proposal.

"I don't think I'll be going up against them," Mr Trump said of the gun lobby. "They're good people."

In other developments:

Media playback is unsupported on your device Media caption As a gunman opened fire in a Florida high school, students inside reached for their phones.

What other issues are attracting boycotts?

The campaign comes as US businesses increasingly find themselves entangled in political debates, as activists target them on issues such as LGBTQ rights, as well as ties to the president.

Companies such as retailer Nordstrom and sportswear brand Under Armour are among the firms that have been subject to calls for boycotts from the left and right.

Executives serving on presidential councils, including the former chief executive of Uber, have resigned from the advisory groups after consumer pressure. The councils eventually disbanded last summer.

North Carolina last year also rescinded a law that restricted bathrooms for transgender people after a boycott by businesses and sports leagues.