British MPs who have long campaigned for Brexit face a dilemma - back an unsatisfactory divorce deal on Tuesday or hold out for something better at the risk of losing everything.

Many "Brexiteers" - once known as "euroskeptics" - have built their whole political careers on opposition to Britain's membership to the EU, and success is tantalizingly close.

But few have any enthusiasm for Prime Minister Theresa May's proposals for leaving on March 29, hammered out over months of talks with the European Union.

They fear the deal will leave Britain a "vassal state" with its plans for a transition period and the "backstop" plan for the Irish border, and chafe at the 39 billion pounds exit bill.

But if they reject it, MPs could vote on Thursday to delay Brexit - giving further time for pro-Europeans to try to undermine the whole process.

Environment Secretary Michael Gove, a leading campaigner for Brexit in the 2016 referendum, admitted the deal was a compromise but urged his colleagues to get behind it.

"None of us Leavers should try to make our perfect Brexit the enemy of the common good," he wrote in the Daily Mail newspaper.

But the eurosceptic Daily Telegraph said it would be "the most humiliating treaty this country ever agreed to."

Some believe a better deal is still possible before Britain's scheduled exit from the EU - or say they are ready to walk away without an agreement.

When May's deal was first put to the House of Commons in January, more than one-third of her Conservative MPs - 118 out of 306 - joined opposition parties to vote against it.

The 10 MPs in Northern Ireland's Democratic Unionist Party, which props up May's government in parliament, also voted it down.

The prime minister has now secured some guarantees on the hated backstop arrangement, intended to keep open the Irish border.

But they fall short of the changes to the treaty text itself that many lawmakers had demanded.

The Brexiteers are not a homogenous group, and there is a hard core who are unlikely to be swayed - enough to scupper the deal.

When May asked MPs last month for more time to negotiate, 20 Tories voted against her because she had raised the possibility of delaying Brexit if she failed.

They include Bill Cash, a rebel who took on former prime minister John Major over the EU's Maastricht Treaty.

May has used various arguments to persuade MPs to back her deal, warning both that defeating it risks a "no deal" Brexit, or that Britain may never leave the EU at all.

Fear of the economic damage of withdrawing with no deal prompted pro-European ministers to threaten to resign if she did not allow MPs to vote to delay Brexit as an alternative.