Britain's Border Force has a "worryingly low" number of boats for patrolling the coast - despite being given a key role in heightened security arrangements, a Commons committee has warned.
MPs highlighted differences between the UK's fleet and the capacity other European nations can call on - and said Royal Navy vessels should be made available to plug any gaps.
They also called for security to be stepped up at smaller ports amid fears they are being targeted by criminal gangs.
Controversy erupted earlier this year when it emerged that just three Border Force cutter vessels were being used to patrol the UK's 7,000 miles of coastal borders.
The issue fell under the spotlight after 18 Albanians were rescued from a sinking inflatable boat off the Kent coast.
In May ministers announced measures to bolster maritime security, including new patrol boats for Border Force to supplement the existing five-vessel fleet. While the first batch were to be in place within months, full deployment is not expected until the end of next year.
By contrast, reports suggest Italy has as many as 600 boats for 4,700 miles of coast.
Border Force has been given a "key role in implementing strengthened coastal security measures" - but it is "experiencing problems in gaining access to a sufficient number of patrol boats", the Commons Home Affairs committee said.
Its report added: "Only four of the new vessels are currently deployed and the remaining four will not be available for more than a year."
Labour MP Keith Vaz, chairman of the committee, said: "Despite maritime security being critical to an island nation, Border Force is operating worryingly low numbers of vessels to protect our borders."
The Home Office said the comparison in the report between the UK and other European maritime countries only includes the number of cutters currently deployed in UK territorial waters and takes no account of the additional military and law enforcement vessels available.
Security "must be tightened" at small ports which are now being used by gangs to move people between the continent and the UK, the committee said.
The wide-ranging report also:
:: Described conditions in Calais migrant camps as "absolutely atrocious";
:: Accused the EU and its member states of failing to anticipate the scale of migrant flows;
:: Warned of a "two tier system" among local authorities in relation to the Syrian refugee resettlement programme, with figures showing some areas received scores of people under the expanded scheme between October and March - while others took in none;
:: Urged ministers to encourage their own councils to take their "fare share" of refugees;
:: Called on the Government to, "as a one off", accept 157 unaccompanied children in Calais who have family members in the UK;
:: Said maintaining the Le Touquet agreement, which allows British border checks to take place on French soil, should be a "priority".
Mr Vaz described Europe's efforts to address the "colossal" refugee crisis as "lamentable".
He also warned that the Paris attacks "demonstrated that terrorists are exploiting this crisis" and said the EU external border must improve security - including deploying equipment to fingerprint and check everyone against security databases.
A Home Office spokeswoman said: "Our priority is to offer humanitarian support to those most in need while maintaining the security of our borders."
She said refuge has already been provided for more than 1,800 Syrians under the scheme, while the Government is "on track" to deliver on its pledge to resettle 20,000 by the end of the parliament.
The spokeswoman added: "We have also made clear our commitment to bringing very vulnerable children from Europe.
"At the same time we continue to work tirelessly to maintain the security of our border, intercepting attempts to enter the UK illegally and targeting the callous gangs that profit from people smuggling."
David Simmonds, of the Local Government Association, described the report as "out of date", adding: "We are confident that there will be sufficient places that will support the Government's pledge to resettle 20,000 people by 2020."
Mark Serwotka, general secretary of the Public and Commercial Services union, said: "We have said for years that the Border Force doesn't have enough resources, but successive governments have continued to cut budgets and staff.
"Instead of planning humanitarian solutions, the Tories' response to the refugee crisis has been appalling.
"If people fleeing war and desperate circumstances are seen as a problem to be tackled, rather than human beings to be helped, this risks fuelling the kind of anti-migrant and racist sentiment that was stirred up during the EU referendum campaign."
Steven McIntosh, of Save the Children, said: "Making sure our borders are secure is important, but the lives and safety of children must be our top priority.
"The UK Government's offer of a home for lone refugees was a humane and life-saving promise which must be delivered, by ensuring local councils have the resources to support vulnerable children as soon as possible."
He added: "Heads of state have an opportunity to rectify this at President Obama's displacement summit next month."