The Queen thanked the nation for its kind messages on the day she became the longest reigning monarch in British history.

It was business as usual for the 89-year-old monarch, with the Duke of Edinburgh at her side, as she spent the day travelling on the Scottish Borders Railway with Scotland's First Minister Nicola Sturgeon.

As she performed the official opening of the new rail link, said: 'Inevitably a long life can pass by many milestones. My own is no exception. But I thank you all and the many others at home and overseas for your touching messages of great kindness.'

The milestone is tinged with sadness for the monarch as the calculation of the length of her reign is linked to the death of her father, King George VI, and that of Queen Victoria.

There was no mention of Victoria in the Queen's address but she was wearing her great-great grandmother's diamond bow brooch, which was handed down to her through the generations.

The Queen began her speech by remarking that she and Philip were delighted to be back in the Borders and to have arrived by train.

'It's been wonderful to witness the excitement which the return of the railway has brought here,' she said.

Earlier she was greeted by delighted crowds at Edinburgh's Waverley Station as she stepped onto the polished steam locomotive Union of South Africa with Philip.

The Queen, the Duke and Ms Sturgeon travelled in a 'Pegasus' Pullman coach that seats 14 in the main lounge area and also has a bar - that was serving tea and coffee.

The trio were seated in a pristine private compartment at the end of the coach's lounge area with the Queen's entourage in nearby seats.

Tributes have been paid to the monarch with Prime Minister David Cameron declaring in the House of Commons: 'The Queen is our Queen and we could not be more proud of her.

'She has served this country with an unerring grace, dignity and decency and long may she continue to do so.'

Ms Sturgeon in her own speech greeted the Queen with 'a simple but heartfelt thank you' for her work.

She said: 'We are privileged ma'am that you have chosen to mark today's milestone here. All of us are delighted to be able to share some of this special day with you. For those watching from around the world let me say, on their behalf ma'am a simple but heartfelt thank you.'

The rest of the royals carried on with their scheduled diary of engagements, with the Queen's daughter-in-law the Duchess of Cornwall describing the milestone as 'marvellous' on a tour of the studios of ITV show This Morning.

The Queen's son the Duke of York, in a pre-recorded interview with the BBC, spoke of her consistency and leadership.

'It's a milestone in UK terms but as far as her consistency and leadership - it's the normal run of the mill sort of date. It's just one day in her reign,' he said.

'So yes it's an extraordinary achievement in some respects, but actually it's about the consistency and the leadership that she is showing and has shown throughout her reign that I think is probably the one thing that marks her up more than anything else.'

The exact moment the Queen passes Victoria's milestone is not known as there is no precise time for the death of her father who died in the early hours of February 6 1952.

But it is thought he probably died at 1am so at around 5.30pm the Queen will make history when she will have reigned for a few minutes longer than Victoria.