An anti-terror training exercise will be carried out in London over the next two days by Britain's security and emergency services David Cameron has announced.

He told MPs the training would ensure the UK was prepared to deal with a terrorist attack after last Friday's massacre by a lone Islamic extremist in Tunisia killed at least 18 Britons dead on a beach resort.

Downing Street said it expects the number of British victims to grow to "around 30" in what is the biggest terrorist attack on British nationals since the 7/7 bombings.

Tunisian medics carry a woman on a stretcher in the resort town of Sousse

Mr Cameron, addressing MPs after they observed a rare minute's silence in the House of Commons, said: "Over the next two days our security forces and emergency services will conduct a major training exercise in London to test and refine the UK's preparedness for dealing with a serious terrorist attack."

The two-day training exercise had already been planned in the wake of the Charlie Hebdo murders in Paris in January, Downing Street said. The exercise will "test the level of preparedness in the UK" to deal with a similar attack.

Asked whether it would cause disruption in the capital, the Prime Minister's spokeswoman said people will be aware it is going on but should not be affected by it.

The Prime Minister also announced that a national minute's silence will be held at midday on Friday to remember the victims of the deadly attack that killed at 38 people.

Addressing what advice the government is giving British citizens travelling or holidaying in Tunisia, Mr Cameron said the Foreign Office was not advising against travelling to the country, insisting the UK "will not give up our way of life or cower in the face of terrorism".

He said UK tourists were not being advised to stay away from Tunisia's resorts but said these were "difficult judgements" because "nowhere is without risk from Islamist extremist terrorists.

"The Foreign Office has updated their travel advice, which continues to make clear the high threat from terrorism in the country, just as it did before Friday's events," he said.

"But they are not moving to a position of advising against all but essential travel to this part of Tunisia.

"So they are not advising against visiting the popular coastal resorts. This was agreed by the Cobra emergency committee and will be kept under close review."