Such was its size that small vehicles would be able to travel within the tunnel, an AFP photographer granted access to it reported.

The underground passage formed part of a wider branching network that stretched for more than 2.5 miles (4km) and came within 400 metres of the Erez border crossing, the army said.

Israeli forces said it would have cost millions of dollars and taken years to construct, and the project was led by Mohamed Yahya, the brother of the Hamas chief, Yahya Sinwar.

The honeycomb of passageways features drainage and sewage systems, electricity, ventilation and a communication network as well as rails. The floor is compacted earth, the walls are reinforced concrete and the entrance is a metal cylinder with 1.5cm thick walls.

The army released footage that it said had been filmed by Hamas showing a small construction vehicle being driven into the tunnel, an extensive temporary warehouse filled with pre-cast concrete for lining the walls and workers digging beneath the earth using crude power tools.

The army said it had found a large number of weapons stored in the tunnel, ready to be used in an attack.

Lt Col Richard Hecht, an army spokesperson, said Hamas had expended huge resources on the project. He said the tunnel had been deliberately built near the Erez crossing, which Israel uses to facilitate the strictly controlled entry of Palestinian workers and those travelling for medical care.

“For Hamas, attacking the people of Israel continues to take priority over supporting the people of Gaza,” he said.

Hamas launched a surprise attack against southern Israel on 7 October, killing about 1,140 people, mostly civilians, and taking about 250 hostages, according to the latest Israeli figures.

In response, Israel set out to destroy Hamas and launched a relentless bombardment and ground invasion of the Gaza Strip to achieve that goal. Gaza authorities say Israel has killed more than 18,800 people, mostly women and children, during the war.

The labyrinth of tunnels beneath Gaza, which the Israeli military has nicknamed the “Gaza Metro”, was initially devised as a way of circumventing the crushing Israeli-Egyptian blockade, in place since 2007.

Hundreds of tunnels were built under the border with Egypt and into the Sinai desert, allowing people, goods and weapons to cross into Gaza from the outside world.

Since the 2014 war with Israel, the tunnel network has been expanded and Hamas has made frequent use of it to facilitate its rocket launches.

A study published on 17 October by the Modern War Institute at the US military academy West Point said there were 1,300 tunnels stretching more than 500km (310 miles).

The Israeli army said at the beginning of December that it had discovered more than 800 tunnels and 500 had been destroyed.

Reports last week said the army was considering flooding the tunnels with seawater pumped from the Mediterranean and had conducted successful tests.