The Labour MP for Hove, Peter Kyle, told the House of Commons that urgent action was needed with only seven armed officers on duty in the county.
Calls for help to neighbouring forces had increased by 43%, he said.
The prime minister said it was "possible" the pool of armed police could be grown, but he did not want to see "routine arming".
Mr Kyle said: "Can the prime minister assure the people of Brighton and Hove, who have a long history of dealing with terrorism, that should another terrorist attack happen the local force can cope without calling on neighbouring forces?"
The prime minister responded: "As for forces sharing between each other and going to each other's aid, that has always been part of the way British policing has worked."
Brighton's Grand Hotel was bombed by the IRA in 1984.
Five people died and 34 were injured in the attack aimed at prime minister Margaret Thatcher and her cabinet.
An MP has warned David Cameron there are not enough armed police officers in Sussex to tackle terrorist attacks.