* UK PM May expected to win, though poll lead has shrunk
* After attacks, May pledges to crackdown on militants
* Five opinion polls expected on Wednesday
* UK election begins at 0600 GMT on Thursday
By Alistair Smout
LONDON, June 7 (Reuters) - Britain entered the final day of campaigning ahead of a parliamentary election that will define its approach to leaving the European Union but has been overshadowed by two militant attacks in as many weeks.
Prime Minister Theresa May unexpectedly called the June 8 election seven weeks ago, seeking to boost her parliamentary majority ahead of the start of Brexit negotiations and to win more time to deal with the impact of the EU divorce.
But the campaign has seen a number of unexpected twists, including the deadliest militant attack in Britain since 2005 and a sharp contraction in May's once commanding lead of over 20 percentage points in opinion polls.
Attacks by Islamist militants in Manchester and London threw the spotlight on security, while May was forced to backtrack on a social care policy pledge in a move that pundits said was unprecedented in British election campaign history.
"Give me your backing in the polling station tomorrow to battle for Britain in Brussels," May said. "Get those negotiations wrong and the consequences will be dire."
May has repeatedly said only she can deliver the right deal for Britain and that opponents would lead its $2.5 trillion economy to ruin in the negotiations with the EU.
Pollsters expect May to win a majority.
But if she fails to beat handsomely the 12-seat majority her predecessor David Cameron won in 2015, her electoral gamble will have failed and her authority will be undermined both inside her Conservative Party and at talks with the 27 other EU leaders.
When May stunned political opponents and financial markets by calling the snap election, her poll ratings indicated she could be on course to win a landslide majority on a par with the 1983 majority of 144 won by Margaret Thatcher.
But May's poll lead has shrunk over the past three weeks. Latest polls put her party anywhere between 12 to 1 point ahead. One projection said she would win a majority of 64 seats.
There are at least 5 opinion polls expected before polling stations open at 0600 GMT on Thursday.
Opposition Labour leader Jeremy Corbyn, a radical socialist once written off by many as a no-hoper leading his party to its worst election defeat, has run a strong campaign.
May and her husband Philip were greeted with jeers of "Vote Labour" as they visited a London meat market on Wednesday.
The last week of campaigning has been held in the shadow of an attack by three Islamist militants who on Saturday drove a van into pedestrians on London Bridge before heading towards bars and restaurants, slitting throats and stabbing people, killing seven people and injuring dozens.
Corbyn has put the Conservatives on the back foot over the issue of security, critcising May for a drop in police numbers in her time as interior minister. May hit back with a pledge to crack down on Islamist extremism and strengthen police powers.
"If human rights laws get in the way of doing these things, we will change those laws to make sure we can do them," May said in an interview with the Sun newspaper, which endorsed the Conservative Party.
Two of the three London Bridge attackers were known to authorities before Saturday's attack.
Italy said it had flagged Youssef Zaghba as a potential risk after he moved to England last year, while Khuram Butt was known to British security services. Police investigating the attack made an arrest in east London on Wednesday.
Opponents accused May of undermining the rights of citizens for political gain.
"Many people will see it for what it is, which is a rather crass last minute attempt to divert attention from the much more difficult questions around our anti-terrorism policy," said former deputy prime minister Nick Clegg, a Liberal Democrat. (Editing by Guy Faulconbridge and Janet Lawrence)