Ben Wallace accused Labour of presiding over "illegal wars" but Downing Street said he was expressing a "personal view".
There are no confirmed casualties at this time but one Iranian report said 80 "America terrorists" had been killed.
Iran fired a barrage of missiles in the name of Qassem Soleimani, a top military leader assassinated by the United States.
Iran state TV described it as Tehran’s revenge operation over the killing of Qassem Soleimani.
Labour leader to turn up to lead for the Opposition but defence secretary will update MPs
The Defense Department disputed the letter, which came to light a day after the Iraqi parliament voted to revoke the country’s invitation to host US troops.
Boris Johnson's spokesperson, however, insists special relationship remains strong and defends US's right to "self defence".
"We will not lament his death": PM makes first statement after killing of general Qassem Soleimani sparks fears of war.
Tens-of-thousands of people filled the streets for the grand funeral procession.