Mr Levitt had joined three other MPs on a four-day tour of the region, meeting groups and witnessing the effects of the Israeli occupation in Gaza and the Occupied Palestinian Territory on the West Bank.

The incident happened as the group returned from Gaza to Israel, after witnessing the humanitarian work of the United Nations in the beleaguered territory.

When security fears prompted the Israelis to close the border crossing, the party was delayed by 90 minutes. As they finally entered the new compound – across which no Israelis and few Palestinians are now allowed to cross – a loud explosion rocked the building.

"After a few moments' panic we were told to take shelter in the nearby toilet block, which had clearly been built as a bomb shelter," said Mr Levitt.

"No-one had been hurt and no buildings were damaged in the attack. Ten minutes later the all-clear was given.

"Although this was a short range, poorly targeted, Palestinian Qassan mortar bomb it gave us a true flavour of the threat currently faced by people on both sides of the border.

"When the Israelis retaliate – as they did strongly the following day – it is with powerful, well-targeted weapons producing many casualties."

During the visit to Gaza City, the MPs met an 18-year-old boy who had lost both legs in an Israeli attack the previous week, and a 14-year-old who had been wounded in the same incident.

The 14-year-old, who had lost an arm and most of his abdomen, was not expected to live, but had been denied treatment in a more sophisticated Israeli hospital.

"In Gaza 80 per cent of people are unemployed and reliant on food aid," added Mr Levitt.

"Israel has closed the borders and does not allow the import of cement to repair and build homes or public services.

"The sewage system has been bombed and businesses closed by Israeli soldiers even after the settlers have withdrawn from Gaza. The main hospital does not have the drugs and equipment it needs to function anymore. Petrol in Gaza is £4 a litre so there are almost no cars on the roads."