Rishi Sunak was not “straight” with the mother of a Manchester Arena bombing victim, Sir Keir Starmer said as he promised Labour would introduce the law “as a priority”.

However, the Labour leader avoided making a guarantee that his party would bring it to Parliament ahead of the summer recess if he replaced the Prime Minister in Number 10.

Figen Murray, whose son Martyn Hett was among those killed in the 2017 blast, had said she felt “let down” after Mr Sunak told her on Wednesday that he would introduce the legislation ahead of the parliamentary break.

But the PM said his pledge over the law, which would require venues and local authorities in Britain to have training requirements and preventative plans against terror attacks, still stands.

Asked whether Labour would commit to bringing the legislation to Parliament before summer recess, Sir Keir said: “Well, I don’t think the Prime Minister was quite straight with Martyn’s mum when he met her.

“I met her that day as well and I said to her, and I disclose what I said to her, because I’ve asked her if it’s all right to do so, I said if the Prime Minister has committed to getting it through before he calls an election, I’ll 100% back him on that.

“If he doesn’t get it through, we will do it as a priority for an incoming Labour government if we are privileged enough to come in to serve. And I said that to her on Wednesday and I’ll say it again today.”

Ms Murray has been campaigning for Martyn’s Law to be passed and met with Mr Sunak for closed-doors talks about it in the hours before he called the General Election on Wednesday.

She said the Prime Minister had given his word that it would be introduced before the summer recess, but could not promise it would be passed before the next election.

With Parliament prorogued and campaigning for the July 4 General Election under way, Ms Murray, said she felt misled.

But Mr Sunak said on Friday he had not deceived Ms Murray because he could still honour the commitment, telling journalists accompanying him on his whistlestop two-day tour of the four nations: “I said by summer recess and that will still be possible.

“The election is in the first week of July. Parliament will reconvene immediately after that, so there will still be time to bring that law in before summer recess, and that’s what I remain committed to doing.

“We’ve done all the prep work and I had a very constructive conversation with Figen and actually I start by paying tribute, as I said to her at the time, to her bravery in the face of tragedy that happened to her family, to have then campaigned for positive change.”

Ms Murray had walked 200 miles to London to deliver a letter to the Prime Minister calling for his support for the law. She arrived on Wednesday, the seventh anniversary of her son’s death.

She said on Friday: “These last few days have been hard – I was exhausted after walking 200 miles from Manchester to London, I was upset by the anniversary and then I felt misled by the Prime Minister.

“It’s great that the PM is now saying he will support Martyn’s Law if elected – and I thank him for that. But Martyn’s Law was a commitment for the last parliament, not the next one, and I feel let down by all the promises that were made and broken.

“National security is too important to be so low a priority. I hope whoever wins the next election will act immediately to make this right.”