Peace in Northern Ireland “must not be taken for granted”, the UK Government has warned, as MPs marked the 25th anniversary of the Good Friday Agreement (GFA).

Northern Ireland Secretary Chris Heaton-Harris advised caution as he pointed to the decision by MI5 to increase the terror threat level from “substantial” to “severe”, meaning an attack is highly likely.

The move came after a series of incidents targeting security forces in Northern Ireland, including the attempted murder of Detective Chief Inspector John Caldwell in Omagh last month.

The GFA was signed on April 10 1998 and brought to an end three decades of conflict in Northern Ireland.

The 25th anniversary arrives against the backdrop of a failure to restore powersharing institutions as the DUP continues to express concerns over the UK’s post-Brexit arrangements with the EU.

Speaking in the Commons, Liberal Democrat MP Christine Jardine (Edinburgh West) said the debate marked an “important milestone for peace” in Northern Ireland.

She added: “We need to be very careful that we don’t take that peace for granted.

“We have seen the threat level increase recently and increased tension, and it’s as incumbent on all of us now as it was 25 years ago to do whatever we can to protect what is an ongoing process in the peace agreement.”

Mr Heaton-Harris said Ms Jardine had delivered “very wise words” and later said: “The fact Northern Ireland has a locally accountable police force also demonstrates the huge progress that Northern Ireland has made.

“However, I know events such as the abhorrent shooting of DCI John Caldwell illustrate that the peace in Northern Ireland now enjoys and that we’ve all worked so hard (for) cannot and must not be taken for granted.

“I made the announcement that the Northern Ireland-related terrorism threat level has been increased by MI5 from substantial to severe.

“Coming ahead of the agreement’s 25th anniversary this news is particularly disappointing. However, it does not distract from the fact that Northern Ireland remains markedly more peaceful and reconciled than it was in 1998.

“That is a testament to the people of Northern Ireland themselves, as well as the PSNI (Police Service of Northern Ireland) and security services that do so much to keep us all safe.”

DUP MP Jim Shannon (Strangford) said there has been violence in the last fortnight, including “attacks on houses, discrimination and intimidation of people having to move out”.

He said: “It’s only a matter of time before that level of violence spills into either injury or death.”

He added: “The lesson we can take from that 10-year period of relative stability in 2007 to 2017 is that it is only possible to make progress when we fashion an environment that both unionists and nationalists can buy into.”

He went on: “As I look to the future, I am very clear the greatest threat to peace arises from the threat to the Good Friday Agreement, and we should be in no doubt that the threat is now acute.”

He also told MPs: “The UK Government now has a choice… I for one hope that it will learn the lessons of the 2007 to 2017 period, and going forward ensure the Good Friday Agreement, amended by the St Andrews Agreement, is upheld and not ignored.

“If it does not, then for many in Northern Ireland and for myself, I fear for the future of Northern Ireland.”

Mr Shannon also praised the PSNI for doing a “grand job” in his community.

Meanwhile, shadow Northern Ireland secretary Peter Kyle urged Mr Heaton-Harris to give the PSNI “all the support that they need as they face down those who want to turn back the clock”.

He said: “What we must acknowledge is that the police officers have been the focus of recent attacks by dissident republicans. These groups are opposed to the Good Friday Agreement. They attack the police because they want to intimidate those who protect its achievements, its institutions and its legacy.”

Mr Kyle went on: “I hope the Secretary of State will give the PSNI all the support that they need as they face down those who want to turn back the clock on this era of peace and progress.”