British warships, helicopters and several hundred military personnel have been put on standby of 48 hours or less for deployment in or near Iraq, ready to respond as the Iran crisis escalates.

Tensions in the aftermath of the US assassination of Gen Qassem Suleimani led to Iran firing a salvo of missiles on Tuesday night at Al-Asad airbase in Iraq’s Anbar province, which hosts a US contingent, hitting it at least six times.

The backup forces – including a handful of twin-engine Chinook helicopters – could be used to evacuate British troops if the security situation in Iraq worsens further.

The disclosure of the numbers involved is also designed to put Tehran on notice, with the additional details emerging shortly after Ben Wallace, the defence secretary, told MPs that the UK would respond if Britons were killed by Iran or any of its proxies.

“If British civilians were killed or even military personnel as a result of Iranian or terrorist action, we would look at the response. The response would no doubt be proportionate,” said Wallace, as parliament returned from the Christmas break.

Britain has 400 troops in Iraq at present, currently focused on protecting themselves in the aftermath of the controversial killing by an American drone strike that has prompted speculation that Tehran will seek to hit back at western interests.

Defence secretary Ben Wallace addresses MPs on Monday. Photograph: UK Parliament

Two British warships are already stationed near the Gulf, HMS Montrose and HMS Defender, which are ready to escort British flagged oil tankers through the strait of Hormuz if required.

On Friday, Wallace updated MPs for the first time since the killing of Suleimani, and faced questions about whether he believed the drone strike outside Baghdad airport had been legal.

Jeremy Corbyn, the Labour leader, asked: “So does the government regard the assassination as legal under international law? If so, how? And do the lawyers in the Foreign Office and the Ministry of Defence regard it as legal?

“If he really believes this was an act of self-defence, what evidence has he or the prime minister seen of the imminent attack on the US?”

The US has repeatedly said it launched the drone strike that killed Suleimani after receiving intelligence he was planning attacks on American personnel in Iraq, without providing further details.

“From the information and intelligence I have seen it is clear that there is a case for self-defence to be made, about an individual [Suleimani] who had come to Iraq to coordinate murder and attacks on US citizens,” Wallace said.

Johnson was conspicuously absent from the debate. The prime minister has not appeared in public since returning from holiday in the Caribbean over the weekend – prompting Corbyn to accuse Johnson of “hiding behind his defence secretary”.

Wallace hit back, saying: “Funnily enough, the prime minister is running the country, something the leader of the opposition will fail to ever do.” Wallace said Johnson believed cabinet members should answer questions where appropriate.

Wallace also promised to “issue guidance imminently” to oil tankers and other British flagged merchant shipping passing through the strait of Hormuz and said the threat was being reviewed on a daily basis.

Defence sources also said that about 50 British personnel had been ordered out of Baghdad’s secure Green Zone, and relocated for their own safety to either Camp Taji, 24 miles (40km) north of the Iraqi capital, or out of the country.

Meanwhile, about 20 military planners have flown to Iraq to help prepare for a range of scenarios, from an escalation of conflict with Iran to a full departure of British forces from the country. They were engaged in “contingency planning”, Wallace told MPs.

The deployment means that the UK’s overall contribution to coalition troop numbers in Iraq remains at about 400, with the majority stationed alongside Australian forces at Camp Taji.

On Sunday, British forces in Iraq halted their work helping train Iraqi soldiers to counter Islamic State. The soldiers are now focused on “force protection” – guarding themselves against potential local reprisals, particularly from pro-Iran militias.

Meanwhile, Rob Macaire, the UK ambassador to Iran, was summoned to meet the Iranian European affairs minister after Johnson said he did not lament Suleimani’s death and the UK foreign secretary, Dominic Raab, said the UK was on “the same page” as the US.

Following the meeting, IRNA – the semi-official Iranian news agency – reported a foreign ministry spokesman as saying “the people and the government of the Islamic Republic of Iran regarded the positions adopted by the British government constitute an alliance with a US terrorist act. It cannot be justified and accepted, and with such positions Britain can be a partner in this crime.”

Although the remarks do not mean that British troops would be regarded as legitimate targets in any potential Iranian reprisals, the summons underlines the risks UK ministers take in trying to remain politically aligned with the inconsistent Trump administration.

Privately, some British ministers regard Trump’s decision to launch the strike against Suleimani as erratic, but the UK cannot afford to be seen as being out of step with its closest long-term ally, not least when US-UK trade talks are due to commence.

Capita, the firm that handles recruitment for British army, said that they saw the highest number of applications to join in a single day on Monday. The heightened interest is likely tied to the possibility of conflict with Iran. Army recruitment traditionally increases when people believe there is a possibility of war.