* Conservative Party facing "catastrophic split"
* May faces tumultuous party on Sept 30-Oct. 3
* Boris Johnson steps up attacks on May
By Guy Faulconbridge
LONDON, Sept 10 (Reuters) - British Prime Minister Theresa May's Conservative Party faces a "catastrophic split" if she persists with her proposals on Brexit, which 80 or more of her lawmakers are prepared to vote against, a former junior minister said.
Such public criticism, a day after former foreign minister Boris Johnson cast her Brexit plans as "a suicide vest" wrapped around the British constitution, indicates how hard it could be for May to get any Brexit deal approved.
Steve Baker, a former junior Brexit minister who resigned over May's so-called Chequers proposals on Brexit, told the Press Association he was not advocating a change of leader but said May faced a massive problem at the Sept. 30-Oct. 3 party conference.
If 80 of May's 315 lawmakers voted against a Brexit deal based on her proposals, the fate of the government and exit process would depend on the opposition Labour Party because she would not command the 320 votes needed to dominate parliament.
"If we come out of conference with her hoping to get Chequers through on the back of Labour votes, I think the EU negotiators would probably understand that if that were done, the Tory party would suffer the catastrophic split which thus far we have managed to avoid," Baker was quoted as saying.
Other Conservatives have given much more modest estimates of the number of lawmakers who oppose her plans. While some Brexiteers are unhappy with her premiership, they see May as their best immediate hope of ensuring the UK leaves the EU.
The United Kingdom is due to leave the European Union on March 29, yet little is clear. There is, so far, no full exit deal and it is uncertain whether May can push an agreement through the British parliament.
Divisions in the Conservative Party over Britain's relationship with the EU contributed to the fall of all three previous Conservative premiers - David Cameron, John Major and Margaret Thatcher.
While the EU in recent weeks has given signs it is seeking to ease May into a deal by offering compromises, she is facing an increasingly vociferous group of committed Brexiteers who feel she has been far too weak with the EU.
Under May's proposals, Britain will seek a free trade area for goods with the EU, largely by accepting a "common rulebook" for goods and British participation in EU agencies that provide authorisations for goods.
Some Brexiteers say those proposals would ensure the EU kept control over swathes of the British economy and thus run counter to the spirit of her manifesto pledge to leave the EU Customs Union and the Single Market.
In an article in the Mail on Sunday newspaper, Johnson pressed his attack on May's plan, calling it "a humiliation" that opens "ourselves to perpetual political blackmail".
Baker said May should seek a Free Trade Agreement under the terms placed on the table by European Council president Donald Tusk in March, PA reported.
He said it would be "fanciful" to expect her to secure parliamentary approval for Chequers.
"It is absolutely no pleasure whatsoever to me to acknowledge that, but I look at the mood of colleagues and the mood of the Conservative party in the country and I am gravely concerned for the future of our party," Baker said.
London and Brussels say they want to get a divorce deal at the Oct. 18 EU Council or at the latest by the end of the year.
Any deal with the EU must be approved by the British parliament, which is due to go on Christmas holiday from Dec. 20 to Jan. 7. If British lawmakers reject a deal in late December or early January, Britain would face the prospect of leaving the EU three months later without an agreement.
Without a deal, the UK would move from seamless trade with the rest of the European Union to customs arrangements set by the World Trade Organization for external states with no preferential deals.
Some business leaders have warned that adding just two minutes onto every lorry’s customs procedure passing through the southern English port of Dover would produce a 14-mile tailback on either side of the Channel after one day.
Uncertainty over Brexit and its impact on the pound is making it hard to predict the future profitability of Primark, the owner of the discount fashion chain warned on Monday.
"Of all the years that I've sat in this chair, this coming year is one of the most difficult to give guidance on because there is this uncertainty of the Brexit outcome," Associated British Foods finance director John Bason told Reuters.
Supporters of Brexit acknowledge there may be some short-term pain for the UK’s $2.9 trillion economy but say it will prosper in the long term when cut free from the EU which they cast as a failing German-dominated experiment in European integration.
Additional reporting by James Davey; Editing by Michael Holden, Janet Lawrence and Andrew Heavens