'Once bitten twice shy': Arlene Foster warns Boris Johnson he may not be able to count on DUP support after the election as she demands written guarantees there will not be customs checks between the UK and Northern Ireland after Brexit

• Arlene Foster said the DUP is still opposed to the Prime Minister's Brexit deal
• Ms Foster suggested DUP needs written guarantees on customs arrangements
• Mr Johnson has insisted 'there's no question of there being checks on goods'
• But Ms Foster said words were not enough with her party 'once bitten twice shy' 

Arlene Foster has warned Boris Johnson he may not be able to count on the DUP's support unless he provides them with cast iron guarantees there will not be customs checks between the UK and Northern Ireland after Brexit.  

Mr Johnson said yesterday that 'there's no question of there being checks on goods' travelling between Northern Ireland and the rest of the UK or the other way around. 

But there is growing confusion over whether that would be the case with suggestions that the Prime Minister's Brexit deal would see checks carried out on goods travelling from Great Britain to Northern Ireland. 

Ms Foster said today her party would continue to oppose the premier's divorce accord as she said Mr Johnson's statements were not enough to win the DUP's support. 

She suggested her party would need something in writing to confirm Mr Johnson's claims before it would consider backing another Tory government as she accused the PM of breaking his word on Brexit.  

Mr Johnson had stood alongside the DUP leadership at the Conservative Party's annual conference in October and stressed the importance of the Union. 

But the DUP believe Mr Johnson effectively sacrificed the Union in order to secure a deal with Brussels which the party believes will see Northern Ireland treated differently to the rest of the UK. 

Asked this morning if she would take Mr Johnson at his word, Ms Foster told the BBC: 'I think once bitten twice shy we will certainly be looking for the detail of what this is going to look like. 

'That is why in the week leading up to the deal we were engaged with trade officials, we were engaged with officials from Her Majesty's Revenue and Customs to make sure that what was being said was actually factually correct. 

'We will continue to do that.'

Opinion polls suggest Mr Johnson is on course to win a majority at the general election on Thursday. 

But should the polls narrow and the PM fail to win a majority then the DUP and its likely handful of MPs could be key to whether he is able to stay in power. 

But as it stands the party is adamant that it cannot support Mr Johnson's Brexit agreement.    

Ms Foster said: 'It is very important for us in Northern Ireland not just to have the word but to have the detail. 

'We want to see the detail. We did see the detail of that week when we spoke to HMRC officials and they said that this would be what would happen, that there would be checks between the mainland and Northern Ireland, something which will cause economic instability in Northern Ireland which will lead to higher costs for retailers, which will lead to less choice for our consumers in Northern Ireland. 

'All of that leads me to say that we need to, after this election is over, and I am not prejudging the election in any one way, every vote will count, but what is important is that we have a strong team of DUP MPs back in Westminster to speak up for Northern Ireland. 

'Because in the event of a hung Parliament or in the event even of a small majority for a Conservative leader we will be wanting clarity and we will be wanting to make sure that we deal with this customs issue.'

It came as a leaked government document suggested delivering the customs arrangements related to Northern Ireland's post-Brexit relationship with the UK by December 2020 would be a 'major challenge'.